Try checking the Deleted Items bin. Data you've deleted on your own computer is sent here and stored indefinitely, so try emptying this folder. You might also check to see if you have a very large number of historical versions saved. Usually these deduplicate, but some file structures don't deduplicate effectively and your historical versions might be taking up a lot of space.
To delete historical versions, you can run a command to purge them from your account
If for any reason you still think the amount of data you see listed is "wrong", or you think your account is growing larger than it should be, you might have some unnecessary or "junk" files selected for backup. For example: Temporary internet files, cookies, "Recent" menu, and so on. Cookies change continuously and have lots of tiny data values, which cause a huge amount of overhead in the backup process.
These folders would never want to be restored, so there's no point in backing them up. Some SpiderOak users accidentally back up the data files for a different SpiderOak user, which means they store all the unnecessary temp files and application data files SpiderOak generates.
These files change often and accumulate a lot of historical versions, and moreover there's no need to back them up. Try going through your account and make sure you have nothing backed up that you don't recognize. If you don't know what it is, there's probably no need to back it up!