Highly variable backup sizes?

OK, here’s a curiosity question for the MikroTik guys. I’m seeing wildly variable backup sizes.

-rw-------.  1 tjc tjc 169671 Aug  5 21:16 MikroTik-05082011-2112.backup
-rw-------.  1 tjc tjc  57000 Jul  9 19:17 MikroTik-09072011-1913.backup
-rw-------.  1 tjc tjc  30190 Aug 13 17:29 MikroTik-13082011-1728.backup
-rw-------.  1 tjc tjc  56262 Aug 14 13:56 MikroTik-14082011-1356.backup

I did something silly yesterday, which resulted in locking myself out of the router, and ended up with a reset to the default config. (duh-oh!) Since restoring from exports I’ve been trying to keep current snapshots, but the sizes are all over the map, which seems a bit strange.