Art assets I'm guessing, there are more of them and higher quality means they're bigger and more complex.
Plus all the voice acting recording.
This is pretty much it. Most of the DAO textures were 256, with the higher ones being 512 with the rare 1024. In DAI there are some 256, but most are 512, 1024 for some higher quality, 2048 for super hq and many armors, and a very rare 4096 here and there. Also, as you said there is a lot of voice work, including everything for the protagonist -- 4 different voices total for each line! -- which DAO didn't have.
And of course it's just a larger game overall. Bigger areas, more items, including everything for crafting and all the coding that goes with that, more units, more of everything.
There is also quite a bit of music. I know people complained about the ambient environmental music, but it's all there, and each area has its own score. In addition to that, all of the areas have multiple versions of the score to follow along with the pace of action on the screen; there is a casual tempo, a more combat/exciting tempo, a scary tempo, and so forth that are all done as separate themes and loaded as separate sound files.
And yeah, you can compress, but there is only so much compression you can do before quality takes a noticeable hit.
In short, there is a TON of stuff outside of the base scripting (considerable by itself) that is stuffed into the game files.