It's FOUR fucking games, all in HD, with HD audio, all on ONE disc. Two of those games have been remade fully, with entirely new, updated, visual and audio assets. One of those two has brand new, pre rendered cinematics. Let's not forget that those same two games are also playable in their original forms as well. The remaining, other two games feature revamped assets and effects that, yes, require more space. Oh, and of course, all four games will feature their respective multiplayer components too, one of which has six remastered, totally remade maps. Throw in the master UI and it's components and you got the full package.
Tell me how you reduce the size of something like that?