This trope basically comes from a combination of RuleOfCool and BiggerIsBetter. An oversized sword is a unique weapon that will stand out from the crowd, and marks its wielder as a person of above-average or even superhuman strength. Many writers like for their protagonist or villain to be as cool and unique as possible. As for its usefulness as a weapon, most people assume that a bigger, heavier weapon will inflict more damage on whatever it hits, provided that you can find a person strong enough to wield it. Force equals mass times accelleration, so if you swing two weapons of different masses at a target going at the same speed as each other, then the more massive object will hit with more force. However, people might fail to consider that oftentimes it is more efficient to increase the acceleration of the weapon than to increase its size. Modern rifles using energy efficient smokeless propellants shoot smaller bullets at higher velocities than their black powder ancestors, which combined with rifling gives them more accuracy, range, and penetrating power despite not increasing the weight of the actual projectile. To continue this comparison, when you're talking about a gun, the propellant in the cartridge case and the firearm in which the explosion takes place are what propell the projectile, while when you're talking about a sword, it is the human and his or her muscle power which accelerate the projectile. Unlike designing a gun cartridge, where if you create a more powerful cartridge then you can design and manufacture a new firearm whose breech and barrel can handle the increased pressure of a stronger cartridge, you cannot create a bigger sword and then design from scratch a human who is big and strong enough to wield it. And also unlike with guns, where a gun too big to be fired from the shoulder can still be used as a crew-served weapon mounted on a tripod, gun carriage, or vehicle, there is no use that can be found for a sword too big to be wielded by a single warrior. Therefore, sword design has to take into account what the average physically fit human warrior is capable of.
Sheer weight or mass is not by itself a good thing; you need to make the sword just heavy enough in the right places to do the job intended, without getting to the point where the mass is just slowing you down and robbing your strike of energy. Generally you can make a blade big in one or two dimensions, but not in all three dimensions at once. Some swords such as the Chinese ''dadao'' or Philippine ''panabas'' have very short, very broad blades and long two-handed grips in order to get a lot of chopping power like a meat cleaver at the cost of short reach and having a not-very-servicable point. Medieval cutting-focused greatswords would often have a long blade that was broad along its entire length (in other words, little profile taper) so it would have both reach and cutting power as well as a broad point that would do pretty well against unarmored or mailed opponents, but had an aggressive distal taper, meaning that the blade was thick near the guard but got progressively thinner towards the tip, keeping it from getting too heavy towards the point but making it less suited for penetrating hard targets or slipping through the gaps of plate armor with a thrust. And then there's the rapier or estoc, which sacrifices most of its cutting ability for point control, reach, and penetration: The blade is very long (for reach), has a thick spine along the entire length (for stiffness), an accute point (for penetration), and a narrow profile (also to help penetration, and to reduce the weight). There is also more mass in the guard relative to the blade, pushing the center of balance closr to the hand, which makes it easy to manipulate the tip. However, this makes for a far-back center of percussion which reduces the range at which you can make effective cuts, and the thick edge section combined with the light blade reduces cutting power. Each of these swords, despite their differences in handling, are basically agile and maneuverable weapons because they make compromises. When it comes to length, breadth, and thickness, you can pick any two but have to sacrifice the third if you want it to be a handy weapon. What the BFS does is expand in all three dimensions at once, to the point where the overall weight and balance would become unmanageable.
Giant swords are impractical in the real world because of the SquareCubeLaw, which guarantees that a sword that gets too heavy will be slow and cumbersome no matter how strong its wielder is. The sword has to be a relatively small percentage of its wielder's body weight--and indeed a pretty small weight in absolute terms--in order for human muscle power to accelerate and control it at high speed. Even a world-class powerlifter would struggle to use a 15 pound sword for fencing at normal speed, simply because it is exponentially harder to swing a substantial weight around your body at more than arm's length than it is to curl or bench press the same weight close to the body in a straight trajectory, and whatever you can do with it will be much slower in comparison. Eight pounds is about the upper limit for what even a six-foot long greatsword can weigh before it becomes too unwieldly for fencing at speed. What's more, wielding a ''truly'' giant sword such as [[VideoGame/FinalFantasyVII Cloud's Buster Sword]] would require not only SuperStrength, but also a whole set of RequiredSecondaryPowers. An object that large has tremendous inertia, meaning that it is extremely difficult to get it moving from a resting position, and just as hard to stop it or change direction after it gets going. In order to actually exert on the weapon the minimum force needed to accelerate it to the speed of a sword swing, you would have to be able to push off of the ground without your feet slipping out from under you as you step forward, and then the sword would try to yank you forward along with it as soon as it gathers some speed. In order to do this with control you would either have to weigh several times as much as a normal human, or artificially increase your personal gravity, stability, and traction using sci-fi technology or magic. Once you were able to swing it and keep your footing, you would also need SuperToughness in order for your bones, muscles, and joints to withstand forces that would rip a person's arms off. Even if you had all these powers for controlling a BFS through a proper swing, a swordfight is not won--much less survived--by relying on one great swing to fell your opponent. You have to consider how long it takes to recover from a missed swing, transition between different guards or stances, and defend against relentless attacks coming from various directions. At best a superman using a giant sword would be rather sluggish and vulnerable to anything nimble enough to avoid his blows, and unless his super strength also came with unlimited stamina, he would quickly become exhausted and lose the ability to either attack or defend himself effectively. It is far more efficient to give a strong person a weapon only slightly heavier than average, so that they can handle it more effortlessly and nimbly.
Many examples of the BFS that were made in real life are simply not meant for combat. An Early Modern European executioner's sword was designed to chop through a restrained person's neck in one swing, meaning that defense and agility were of no importance. Therefore they were made with long grips for leverage, while their blades were relatively short, forward-balanced, and often without a point. Since they had a practical function, at least these were usually within the outer parameters for a fighting sword. In contrast, ceremonial swords had hardly any constraints on their weight and dimesions. For example, King Henry V of England's bearing sword in the Tower of London was exaggeratedly large in order to look impressive from a distance as someone carried it in front of his procession, and the fact that it was too big and heavy for pracical use didn't matter. You could say that this latter category subverts an important part of the trope, because unlike the fictional BFS which is made because there's someone who can wield it effectively, smiths of old were under no illusions that their giant ceremonial swords were useful to anyone as practical weapons.
Real, practical two-handed blades from Europe such as the German ''Zweihander'' and Scottish Claymore weighed around 5-7 pounds. Compare this to about 2.5 pounds for a one-handed arming sword, and 3.5 pounds for a longsword that could be worn at one's side. The two-handed sword had a very long grip, which allowed greater control over the blade than a shorter hilt; where a one-handed blade's weight was held steady by the hand and wrist, the longer hilt of these swords turned it into an arm motion and created leverage through the distance between the hands. Such greatswords were not used just for sheer power, but for a combination of cutting power, reach, leverage, defensive radius, intimidation, and versatility. The large dimensions and inertia of big two-handed swords made it difficult to take full advantage of their capabilities without [[DifficultButAwesome considerable stamina and skill]], which is why the Doppelsoldner who wielded the ''Zweihander'' in German mercenary armies earned double pay for their expertise.