I have contacted support and they gave me an answer. This was my last reply:

To clarify, you’re considering the total allocated disk space, not the actual disk space used. The uncompressed droplet uses a total of ~6GB (see below). This means that you can only create a high cpu droplet from a snapshot of a $5 droplet. Disappointing.

Filesystem 1G-blocks Used Available Use% Mounted on
udev 1G 0G 1G 0% /dev
tmpfs 1G 1G 1G 6% /run
/dev/vda1 29G 3G 27G 9% /
tmpfs 1G 1G 1G 1% /dev/shm
tmpfs 1G 0G 1G 0% /run/lock
tmpfs 1G 0G 1G 0% /sys/fs/cgroup
/dev/vda15 1G 1G 1G 4% /boot/efi
tmpfs 1G 0G 1G 0% /run/user/0