The difficulty is:

Do you express the resulting file size or the amount saved? “30%” could mean you “30% of the original file size” or “you shaved off 30% of the original file size”. You could denote the difference with a sign, like “-30%” but that still confuses people. We ran both options by a lot of our colleagues.

“x% of the original file size” is the least ambiguous, but who has the space to put that entire phrase in their app :-/