When you say 'my folder is over 7GB in size' I can only assume that you're referring to the /mnt/sdcard/clockworkmod/backup folder? Just want to be sure we're on the same page. It sounds like more than 'a couple' - I have 9 different backups that I've made, and they average just under 1GB (my oldest backup, just after rooting and keeping the stock ROM was around 600MB and my latest backup with BAMF 1.10 is about 1.3GB).
Anyway, it's safe to delete those backups if you don't plan on going back to that exact image. For example, if your latest backup includes everything just the way that you like it, there's no reason to keep older backups since the assumption is that if you restored one of them, you'd then have to add a bunch of stuff to get to the point where you're at now.
As far as what's making them that big - I'm no expert in how system files vs. user data is stored, but if you assume that the basic .ROM that you loaded was about 400MB, and then you've added apps and data to that (let's say another 200MB worth) - well, that's already half-a-gig. Multiply by the actual number of backups that you've made
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