Kopia reports my backup size is 32gb but Windows Explorer says it is only 27gb

I am using KopiaUI to backup my Windows files. As far as I can tell, compression is not turned on. KopiaUI says that the backup size is 32gb, but when I check the actual size in Windows Explorer it says the backup is only 27gb.

I am backing up to an a SMB share hosted on a Debian home server.