Stopped TM backup question

so, I was doing a TM backup today of 15.5gb and stopped it in the middle of backing up at like around 7gb. anyway, before the backup, I had 823gb unused on my TC. I stop the TM backup and now I have 816gb unused on my TC... Makes sense right?

So now, I go to finish the back up and TM is backing up the same 15.5gb of storage. Shouldn't it only be like 8.5gb or something like that? Shouldn't the TM backup pick up from where it left off? All the while my TC is still at 816gb and after the back up, it will be at 800gb unused instead of 807gb unused.

I lost 7gb of storage on my TC. How can I get that storage back?