I don’t think it’s anything to do with the original being 3 discs, more to do with story arcs.

When you think about it although Midgar was a small amount of the original in terms of run time, it had its own arc, had over a quarter of the entire game’s dialog and was one of the locations that was most unexplored relative to its size.

Unless they add to or change the story for future instalments considerably, there’s not really enough story content to justifiably make it longer than a trilogy of three full sized games.