BG3 is a top down CRPG. Having 3D assets and being a 3D game with full 3D movement aren't the same thing.
And whether it's more content is debatable. There's more pure story and production, with a lot of branching, but the overall amount of space (not counting Starfield's use of negative space because of the setting) is significantly smaller. And even in terms of total number of quest lines, Starfield has a lot. Which you can get more time out of is all about personal preference. There will be people with 1000 hours in both, easy.