Some types of files compress better than others.For example, various multimedia files are already highly compressed because the standards for these file types specify efficient techniques to compress the data they contain. Examples include files in the graphics (picture) file formats GIF and JPG, MP3 music files, and MPG movie files. Once a file has been compressed, it typically can’t be compressed again to any significant extent. Therefore, such files don’t get very much smaller when they are added to a Zip file.
You would see similar results if you compressed some files into a Zip file using maximum compression, and then compressed that Zip file into another Zip file. The second archive would not be appreciably smaller than the first one (it might even be a little bigger). This is because the data in the original Zip file is already compressed and can’t be compressed again.
There are other file types that don’t compress well. For example, certain types of encrypted data files such as those maintained by home finance programs and some database products can’t be compressed very much.
By contrast, some types of data (such as text files and picture files in the BMP format that the Microsoft Paint program uses) can often be compressed by 90% or more; some types (such as program files) are often compressed by 50% or so. But if you see files that can’t be significantly compressed, it’s probably because they already contain compressed data or they are encrypted.