Is it normal for a database size to shrink almost 1/3 it's size on it's own?

[XFFA.IR Bot]

I am starting to wonder now if my staff are deleting posts on my forum. I've noticed my database gzip backup size will shrink at least 5mb every 2 weeks. It's current size is around 24mb but was 35mb just 20 days ago. So one day it's 35 mb and around 3 weeks later it loses almost a third it's size.

I don't know what to do because I only have one staff member but none of the moderator logs show anything has been deleted. I have noticed certain threads I thought had more posts in them don't...

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