Disk Inventory X looks like it's PPC only as it was last updated in 2005. I can't imagine it would work in Mountain Lion.

I tried a commercial app (in demo mode) just before downloading the other program I pasted the link to. I wasn't able to find any reason for the extra storage use. All the paths I checked on my share seemed like the contained the right amount of data in them.

I'm really perplexed here. If I was able to confirm that the backup path was empty before re-scheduling the backup tasks I could mostly explain away the issue. But it's still not quite the right amount of data and I know for a fact that the path was NOT empty.

I'll keep looking but at this point I can't think of anything definitive other than copying everything off onto other disks and then reformatting the NAS before copying everything back. Not something I'd really want to risk unless it's a verified/checksummed copy onto another redundant array.