You can totally mainline FF7 in under 20 hours, but what the post you're replying to post is saying is correct in the sense that there are 3 discs and all that, but Midgar is roughly one third of the game script on paper. So in terms of story events, Midgar is around a third. The problem, especially for a remake like this, is that the middle act of the game (in story terms) burns through assets/areas at about 3x the rate of Midgar even though in story terms less happens in each area than any one given area of Midgar.
The challenge on subsequent games is that there's a lot more areas and places, all unique, all with unique enemies etc, and whereas in story terms Midgar lingers a bit in almost every area, the rest of the game moves at breakneck speed, which means it's burning through those assets super quick. So part 2 and 3 (and beyond, if that's what we end up getting) will probably need a lot more padding, story-wise, than the Midgar chapter.
