Compression ratios depend greatly on the content being compressed.
If I try to compress a 100MB text file which is comprised of the word "wibble" over and over again, the resultant archive will be tiny because all it has to do is to _describe_ the repetitive content once - "repeat 'wibble' a million times".
By comparison, if you're trying to ZIP something that's already highly compressed, like JPGs for example, you may see very little benefit or even none at all. In some cases, the archive can actually end up slightly bigger than what you started with because of space needed for the ZIP header/metadata.
