This really helped
Why does my backup take so much space.
iPhone 8 Plus, iOS 15
The total space taken up by a backup file of your iPhone depends on how much data is there to backup in the first place. A direct answer to your question is that the backup takes so much space is because you may have that much data.
Why do you feel that it should take up less space? Can you provide the specifics?
Axel F.