GPUs have been using a 256MB window since they started coming with 256MB of VRAM, as it would otherwise be a pointless waste of address space to have a window larger than the actual amount of VRAM. Previously, they would have a window exactly equal to the size of VRAM (likely rounded up to the next power of 2.)

There would also be a block of memory mapped hardware registers also needing address space that you have to poke to make the GPU actually do GPU things, instead of just being an expensive way to add extra memory to a system, no?

And if my experience from embedded development is in any way transferable, they're probably fairly spread out and probably takes a fairly big chunk of address space too.