The Motif ES Manual has a specifications page that says something like 175MB Wave Rom (when converted to 16-bit linear format). The phrase in parenthesis suggests that the the sounds occupy 175MB when uncompressed, but that less than that in the Motif ES. I don't have a Tyros manual handy, but someone should check the specifications page and see if the same language is present.

I believe the compression reduces the precision used to encode loud sounds since the human ear is less sensitive to differences in loud sounds than it is to differences in soft sounds.