Pfft, we don't even know that these weapons are supposed to be made of (for the most part). For all we know, Ultima weapon could be made out of some ultra-light material that's easy to swing around. And I think it's pretty clear that either people in FF7 are stronger than humans on Earth or that gravity is weaker on their planet (which would make sense, since their planet is small). I don't see anything wrong with the original, huge Ultima weapon.