Analysis / BFS

Giant swords are impractical in the real world because of insurmountable laws of physics and biomechanics. The most obvious problem is that a ridiculously huge sword would be ridiculously heavy. However, 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 sliding, and you would also have to prevent the resulting momentum from yanking you off your feet. 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.

Even if for the sake of argument all of these powers existed, there is simply no utility in having an excessively heavy sword past a certain point of diminishing returns. Regardless of a wielder's strength, an exaggerated weapon would still have ponderously slow handling compared to a lighter one. After a certain minimum you can get more force transmitted to the target by increasing the sword's velocity than increasing its weight would anyway, which is why for the most part this trope is pure Rule of Cool. See here for the full analysis by Skallagrim.