thunderbird 3 very very big inbox size for imap account

[Gianluca Cecchi]

hello,
just seen the warning window about low disk space in my /home file
system (nice introduction in F12 I presume...)
After searching what has been growing I found this in
/home/gcecchi/.thunderbird/pe1wv5qu.default/ImapMail/myimapaccount

# ll INBOX
-rw------- 1 gcecchi gcecchi 22878721204 2009-12-14 16:40 INBOX

almost 23Gb!!!!!!

This is an imap account against an Exchange 2007 server.
In inbox I have less than 1000 messages where less than 10 are more
than 1Mb in size......
So I'm questioning fron where this big size arrives.....

More strangely, I have a mail with 11Mb of size in INBOX.
I delete it and suddenly INBOX size increases of about 14.7Mb!!!!
And in the mean time I have received no mail at all....

# ll INBOX
-rw------- 1 gcecchi gcecchi 22894180950 2009-12-14 16:43 INBOX

Any hint on thunderbird 3 settings?
Thanks,
Gianluca