Qube Private Storage Max Size Capped?

I’m trying out Qubes OS on a 128gb flash drive at the moment. I’m planning on having a “vault” qube with a large amount of disk for holding files. However it looks like the “private storage max size” option for the qube that I made for it seems to be capped at 67.5GB, and won’t let me increase it further. That’s a decent amount of space, bu it’s only half the drive. I’d like to allocate more.

Is there something that’s blocking me from allocating more? How would I determine what is doing that?

The installed OS itself requires a certain amount of space, as well as a certain amount of space allocated to dom0, templates, and every other qube. That might really be how much is available for that particular qube.