If necessity is the mother of invention, the desire to kill and maim our sibling humans is its father. Nothing pushes science and technology further like war, especially when that technology needs to be really, really big. The regrettable parts of history are full of wild-eyed madmen claiming their superweapon will snatch victory from a devastated foe, and whatever form that superweapon is supposed to take it's almost always stupidly huge. Even crazier are the times somebody actually built one of these uberweapons, and a few of them are even in use today.
Sometimes a weapon needs to be bigger to take more punishment or increase its killing potential, but a surprising amount of the time it's the need to simply look terrifying that increases size, weight, calibre and wingspan. Often the need to just prove we can make something that big is what drives up dimensions and cost, and once we've built it, well it would seem rude not to actually use the damn thing.
Some people just seem to be compensating for something, and an alarming number of times it turns out they were the ones at the drawing board.