ITunes backup size

Hello,

My IPhone 7 has 256GB memory size.

I use a PC and ITunes for backup.

When I connect the phone to the PC, the size of DCIM folder (photos and videos) is about 163GB.

I executed a full backup from ITunes, and the resulted directory size is about 109GB.

I DON'T use iCloud for photos, it is off in Settings.

My question: as I am convinced that all DCIM content should be included in the backup directory, why there is such a big difference in the size of DCIM folder and backup folder.

I also manually copied DCIM folder on the PC and I zipped it to see how effective a compression can be, and the resulted zip file had about 131GB, still far from 109GB.

ITunes uses such a good compression when creating the backup that squeezes 163GB into 109GB or there is another explanation for this?

Thanks for the help!

George