you can do that by adding the following line before save
zip.CompressionLevel= Ionic.Zlib.CompressionLevel.BestCompression;
Alternatively you can refer to following link for detailed answer Compression issue with large archive of files in DotNetZip
Pl. Note: the Library has limitations in compressing image files, the above link will give you a clear understanding of whether the file is getting compressed or not. You can also refer http://dotnetzip.herobo.com/DNZHelp/Index.html for additional help on this topic