To be honest I'm not quite sure. I've noticed sound always seem to take up a large chunk of the project, but as to why the .dat format I will have to admit that I actually haven't looked into the details of audio that much.
What I can tell you is that I think the music tracks are quite large, and were uncompressed as .WAV when I added them to the game. The game is made with Game Maker Studio 2, so maybe their audio system turns it into .dat for reason. I do not know!
Thanks for checking out the game!