Image file uses more space then allowed

So, I made a cache with 2 ssd's of 120 GB (112GB irl)

I made a VM with a 100GB img file.

When I put the VM on the cache, everything is fine.

When things are being written to the image the cache fills up but the img file is still 100GB

If I move it to the array and back to the cache I can use it again until the remaining 12GB  get's written again.

The img file is the only thing on the cache:

/dev/sde1        112G  112G   72K 100% /mnt/cache

 100G -rwxrwxrwx 1 root   users 100G Jul 22 10:01 vdisk1.img*

Can anyone explain to me why this is happening and how I can stop it?

thanks!