MySQL backup size is constantly growing

[d40sithui]

hey guys,
weird issue. i run a small website with mysql backend. since yesterday when i do "mysql -u username -p dbname>backup.sql" the file size is enormous. It is usually 1mb in size. and was so since april 25. For some reason, yesterday i ran another backup and it came up to over 4mb. today it is over 5mb. what could cause this? i'm checking the sql files now.
also server logs indicate this user accessing the site A LOT.

65.55.104.156 - - [29/Apr/2008:16:13:30 -0400] "GET /lan/index.php?name=News&file=article&sid=133 HTTP/1.1" 302 - "-" "msnbot/1.1 (+http://search.msn.com/msnbot.htm)"

[d40sithui]

anyway, so i found out that the cms that this site is on stores session info in a table. so whenever someone accesses the site, an entry is posted. however, for some reason, it doesn't clear the data as it use to. i'll have to search further.