I searched around and found the problem-- I had an artificial limit set on my default email address, not realizing that it also imposed that limit on my site space also!
So I changed it so that there was no limit and saved the changes. The status read, "changed" for a while, then turned to "active". But the problem is still apparent, an error that I've exceeded my storage quota!
I verified with my Disk Usage panel that I'm only using a quarter of my plan's storage space, what gives?!?
I thought that when the change reads, "active", that the change has taken effect. Is there something else going on or is the delay longer than I thought?