Why iZip some times does not reduce the file size much?

iZip uses a method called "lossless" compression, which means you lose no information when compressing files into a zip file. 

How much it can reduce is based on what type of files are compressed. The ratio varies. For example, for video and MP3 files, the reduction ratio is low because these types of files already have very good compression and room for further reduction is minimum. If you compress text files, you will see higher reduction ratio. This is not an issue with iZip, instead, it's the common problem for all lossless compression tools.