I would guess that most of that is textures.

They could probably reasonably just ship half-resolution textures on disc and make higher-res textures be a downloadable option. Bethesda did that with their games, later on added free, downloadable higher-res textures. Shouldn’t require a lot of technical work. Maybe reducing texture resolution could trip some existing bug, a race condition somewhere, but it should be pretty mechanical, as changes go.

I don’t really have a dog in this race—I wouldn’t get the game either way. So I’m not pushing for that as an option. Just saying that I think that it’s an option from a technical standpoint, if one wanted to provide a compact, on-disc version of the game.