I have the same problem and I think it’s because of the music I loaded onto the phone.
The problem is, if you already backed up at least once, you can pick and choose which apps not to back up and thus save space. My 256gb iPad only takes less than 100mb to backup.
However, you don’t have access to that menu if you have never back up.
If I want to fix this I have to either delete the music app, and thus reload them later. Or I have to pay for more storage, make the backup smaller, and stop paying.