It's not isometric though, the camera can be controlled, zoomed in/out/rotated, and it has a full 3d world. And it's huuuuge. Don't get me wrong, I don't think any game should be that large, but BG3 has at least some justification for it.

I think it's probably the gargantuan amounts of super hi definition audio that do BG3.

it's a game with an insane amount of dialog and narration, with branching stories. that's a lot of audio. people underestimate how much voice acting adds to the size.

also this is not a old-school isometric game with prerendered assets converted to 2d backgrounds and sprites; it's fully 3d, and it uses closeup angles for dialog and cut scenes so the textures should be geared towards that while regular isometric games can get away with lower textures because they keep the camera distant from the assets at all times.

Is it maybe voice files etc for all the potential branching storylines and conversations that can happen? It's such a spiderweb of branching storylines that I'd imagine it can take up a fair whack but I genuinely dont know jack shit, just spitballing.