Hello I am a private QA tester intern. I have played the game to some extent so here's something about the '''LARGE''' postgame.

USUM's story is divided into two parts, more like a Fire Emblem's 2 chapter style

First part is like SM, but at a faster pace
-VS Seven trials
-VS Part1 'E4' - Faba, Guzma, Lusamine and URS
-VS 'Champion' Necrozma

The conflict of Necrozma is resolved here. That's why GF calls chapter 2 postgame.

Then we proceed to chapter 2 - takes place in megalopolis.it's actually big, you'll see why it doesnt use much space in rom. The conflict here revolves around 'another pokemon'
-VS Seven UBs
-VS 'that pokemon'
-VS E4
-VS Champion