Rather the format for esm and esp files, but yeah. The format they use allow to build very large game worlds with fairly small amounts of data space, it's very efficient that way. For other games the game world data generally take up a lot more space.
BSA files aren't really anything special, more like an uncompressed zip file or something. It's just something they use to bring a
lot of smaller files (which are already compressed) together to one larger file (as it's quite a bit faster to just deal and read one large file compared to many small ones).
The main thing that take up space for Skyrim are the voices and the textures, and they can both be compressed quite a bit. Especially the textures as they aren't really that large to begin with for the console versions due RAM limitations. The 3GB texture patch raise the RAM requirements for the PC version tremendously.