This. 
btrfs uses copy-on-write and inline compression. virtualbox images cannot deal with that.
There are two possible solutions, but you (@Jaqueline) will have to recreate the disk images.
The first solution would be to set the C and m file attributes on the newly created (and thus empty) disk image files to prevent compression and copy-on-write. 
sudo chattr +Cm <name-of-image-file>
The second solution would be to create your image files on an ext4 or other traditional filesystem — e.g. xfs, jfs, et al.