MP3s are used for two reasons. One, the files are compact so are smaller than, for instance an MP3 is about a tenth of the size of a *.wav file playing the same recording. Secondly, they are fairly universally accessible without having to use a specialist programme. A lot of sites stream music using Real Audio, which is another small sized file, but that ties you into a very limited number of players which handle the file format.
The bigger a file, the longeer it takes to download.

Quack!
GtD.