If i remember correctly, they will add all 5 parts of FFVII into this remake. That is why it is so big
They never said this. FF7 remake is only FF7. The only difference is that stuff from the other games will exist within the context of the game's story and not feel like they were added after the fact (like in the original compilation). But it is not these games being added, merely references to events etc.
The reason the game is so big is because environments are being made bigger. and fully explorable in a 3D space (such as how Midgar seems to be expanded in the footage we have seen)