VirtualBox: shrink Clonezilla disk image

I've spent some time until I finally found out how you can not only increase the disk image but also how to shrink it.

Use case:
I had an disk backup created with clonezilla. The source disk size was 1TB and the restore disk size was 256GB. The actual data amount was around 120GB, so it would definitely fit on the smaller disk.
But with clonezilla, I wasn't able to restore it.