1GB is space for almost half an hour of 5000kbps ("YouTube Premium") video. Just what kind of eye candy are we talking about here?

It's also only 45 frames uncompressed at 4k. It's remarkably easy to hit that if your base assets are mostly raster rather than vector for a dynamic scene.

Obviously, they should try harder, and this is an explanation rather than an excuse, but the graphics assets are mostly why.