Why Does Storage Usage Keep going up on main site?

I have storage space monitoring enabled on all sites, and I notice on my main site it seems to be going up 25 MB or so at a time every day, even though I am not really uploading very much at all. I have added plugins and themes, but it seems that this number goes up even when I don’t add anything. Is it related at all to what is happening on subsites? Right now it’s at about 350 MB. I do have quite a few WPMU plugins, about three dozen themes, and a few other non-WPMU plugins installed. I am sure this could easily account for that usage, but as I say the number seems to go up even on days when I add nothing to the site. Exactly how is this number arrived at?