Well could it somehow be set to incremental?

Huh? No, I posted how it works. Windows Backup does a full backup, then differentials - backing up just the changed or new files since the last full backup. And regardless, Windows Backup is designed to use no more than 30% of the disk.

There is no real advantage to incremental backups over the differential backup method Windows Backup uses. The maximum amount of space used is still the same. Incrementals are great for corporate networks with backup servers and where you have an IT specialist on board. But for home users, doing a full restore from incrementals is a real PITA compared to differential backups, or full image backups.

This is not a settings problem - if it is even still a problem. Something was corrupt and hopefully is not now.