This was the same problem I had with the XL Fuel tank: increasing fuel capacity without increasing weight or drag doesn't make any sense and makes it too easy to make rockets that can travel to the Andromeda galaxy without refueling. My suggestion: rather than have the con be only a higher chance of exploding, increase the weight or increase the amount of drag it causes. Since your idea is that it somehow holds more fuel for less weight, make air resistance a much harsher mistress when this puppy's added on. It could make sense that way; the fuel tank's designed in a much less aeronautically-helpful shape to hold more for the same weight.