Analysis / BFS

This trope basically comes from a combination of Rule of Cool and Bigger Is Better. 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 made a blade big in one or two dimensions, but not in all three dimensions. 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 reach. Medieval cutting-focused grete swerdes would often have a long blade that was broad along its entire length (in other words, little profile taper), 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 good at thrusting against hard targets. And there's the rapier or estoc, which sacrifices cutting ability for thrusting by having a very long blade (for reach) with a thick spine along the entire length (for stiffness), but a narrow breadth from guard to tip which doesn't put much meat behind the edge at the center of precussion. Length, breadth, and thickness: pick two. 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 Square/Cube Law, which guarantees that a sword that gets too heavy will be slow and cumbersome no matter how strong its wielder is for a human. The sword has to be a relatively small percentage of its wielder's body weight in order for their muscle power to accelerate and control it at high speed. Even a person who can lift a 200 pound barbell over their head would struggle to use a 20 pound sword effectively, simply because it is a lot harder to swing a weight around your body at arms' length than it is to slowly raise it straight up and down over your center of gravity. Eight pounds is pretty much the upper limit for what even a six-foot long greatsword can weigh before it becomes too unwieldly. Even so, actually wielding a giant sword such as Cloud's Buster Sword would require not only Super Strength, but also a whole set of Required Secondary Powers. 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, and then the sword would try to yank you forward along with it when it got up to speed. For that you would either have to personally 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 Super Toughness in order for your bones, muscles, and joints to withstand forces that would normally rip a person's arms off.

Realistic two-handed blades, best exemplified by the German Zweihander and Scottish Claymore, weighed around 5-7 pounds, just over twice that of a one-handed arming sword, with a very long hilt, 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 for sheer power (even if they had plenty enough power), but for their's hard to challenge the combination of maneuverability, leverage, reach, agility, and versatility such blades had, and just as nearly as difficult to put it to good use, hence the great deal of fencing styles based on the Longsword relative to all other weapons.