Disclaimer: These opinions are my own and not WT’s, Asobo’s, or Microsoft’s.
The answer to this is very hard fought scores of small optimizations, hiding in tiny unseen corners, diligently researched and tackled over a ton of dev hours, which while each one is really tiny, add up to enough to make these sceneries fit into memory.
The reality is that a ton of these sceneries have but a single LOD, huge textures for small details where not necessary, inefficient UV packing, etc. Just scenery download size doesn’t necessarily tell the tale of how big it will end up in memory. And while I completely understand the level of work involved to do all those things and the very dicey economies of making sceneries, the reality is that many of these would never be accepted as console assets in a game studio environment. And it speaks volumes about the MSFS team that effort is continued to be expended to improve the play experience for folks even though it’s up against this, because they know it’s going to be hard for all scenery developers to meet this, and really everyone on the team just wants everyone to be happy.
Again, just my opinion, but it’s super cool to see some of these having a real effect in these latest updates.