1: Back up the spaces, and then restore them, and delete the original
spaces, and re-invite your space members. THis way, everyone gets their disk
space back.
2: Remove the space from your computer, and get re-invited. Only you get the
disk space back.
Mark is right these are really the only two ways to shrink disc size
manually. The size of the spaces will decrease as space members receive the
commands for deleting the data in the space. So another thing you can do is
to uninvite members from your space that are no longer participating in the
space.