I wrote this elsewhere but I felt it was worth saying again. "9x smaller" is an idiom, much like "spill the beans" or "it costs an arm and a leg".

Idioms don't have to make literal sense or be linguistically/mathematically correct to be useful. All that matters is that other people know exactly what you mean when you say it.

And, pretty much universally, if I tell someone "the compressed file is 10x smaller than the original", they are going to know what I mean is that the byte size is 10% of the size of the original.

That makes it an idiom that is perfectly okay for everyday use.