The graphics are significantly better. This means higher-res textures and models, which already make up most of the space usage in X/Y/OR/AS.I'm curious as to what do these carry. It's not like you have access to other regions, islands are huuuuge or there's many stuff to render at once...
MH4U is understandable, that game is HUGE (2.6) and has at least 90 different monsters+10(more or so) stages.