The SSD in the mini 9 should have a partition as big as the actual size (currently limited to 4GB)
Regardless of the size of the SSD (mine is a 16GB), the disk has a 4GB partition where the Ubuntu OS is installed. The rest is not even formatted. This is a BIG bug, because for the partition to be resized it takes actually a high level of expertize. The unoccupied space can be formatted as an additional partition, but the original is simply to small to accommodate more than the bare minimum. But this is besides the point. If I buy 16GB of space I demand all of it to be available with no further action.
Please see john_h's comment