How do you add custom music without a huge file size?

[Solasun]

I downloaded some songs from Youtube to replace the game's default radio music. Each one is a pretty big wav at about 40 or 50+ MB, which means packing the songs into a ba2 using the Creation Kit will give me an enormous file size. My question is, how do other custom music mods like More Where That Came From squeeze in over a hundred songs while keeping the file size at 106 MB?

 

I've already tried converting the wav files to xwm, renaming the extensions to wav so that the CK can open them. But when I pack the files into a ba2, the size of each song goes back to 40 or 50 MB, making the mod's size close to 800 MB. There is nothing else added into the ba2 (not at this time anyway, as I'd like to get past the file size issue before adding anything fancier).