Since slapcat's ldif is pretty human-readable, you should be able to manually verify what's going on. (use head/tail -somelines is 4.5GB is too big to view) This should give you a good idea of a path to figure out the changes between your last normal backup, and this huge one.
_Matt
Diff can be your friend too. You can diff the two ldif files and get a really clear picture of what changed...
diff old new | less
Owen