Entire library sync
I just set up Zotero Storage and have set my entire library to sync. It says it's syncing, but I have only used 500MD of my storage space. On my computer the Zotero storage folder is over 4GB. I have set it to sync full-text content, so I'm not sure why it is so much smaller.
a) Zotero really needs to do something about (and they know that) and
b) would shrink dramatically on the server, where they are zipped
One other thing would be that if you have groups that you belong to but not own, they'd show up in your folder size but not your online storage usage.