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 :-/

In general, percentages are confusing: depending on the context, "30%" can mean a ratio of 0.3, a ratio of 0.7, or a ratio of 1.3. Of course a large part of the general public doesn't understand that if you make something 10% bigger and then make it 10% smaller you don't get back to where you started, nor that travelling 10% faster won't cut your journey time by 10%, but I've also seen people with PhDs get confused by percentages when discussing compiler benchmark results.

If your main goal is to avoid misunderstanding (which it isn't always) then in my opinion it's best not to have anything to do with percentages. Instead, just give the ratio. It might not be elegant, and perhaps some people won't understand it at all, but "0.7x compression" cannot, I think, be misunderstood. (Or can it? I wait to be corrected!)

30% smaller than what?

Yes, that is Jake. This is Surma.

You seem to be right. Gonna fix the CLI (the CLI is my doing)