How to reduce size and size on disk?

This is probably related to your disk cluster size. Files can be no smaller than the cluster size, which is usually somewhere in the range of a few KB. For example, if your cluster size is 4KB then a 1 byte file will still take up 4KB on the disk. Generally this is more noticeable when you have many small files. If you want to change this you will need to reformat your filesystem and choose a smaller cluster size.