The first 1/2 of Dragon Age takes longer to complete than the entirety of FF7. Lots of cutscenes and complex animations in that game. If you really think updating source material with 40 hours of content is going to result in THREE episodes you are on wild drugs. Final Fantasy 7 isn't "wide" open chock full of content and a ton of sidequests. Even with a massive graphics overhaul it isn't going to be some 100 gig monster. 60 hours?
Even the most technical of PS4 and Xbox games haven't been on the scale of what you're dreaming. The only way is if Square is adding in material that's on the size of another 2 games.
What 'unique assets' in FF7 are taking up space of the magnitude that are gigs worth of data? There is nothing in FF7 that is going to take up that much space. The game is not long at all, especially compared to modern RPG standards. The biggest content on games are typically the engines, textures, and models. CGI and videos are next in line and none of that is approaching 25 gigs on current games. If the original FF7 was 40-60 hours of content then what we'd be getting is a ton of movies and cutscenes and little gameplay. That's the only other way they'd fill out 75 or 150 gigs worth of content short of making new games' worth of material. The OG FF7 having 3 discs has nothing to do with it.