Thunderbird leftover

[yvan]

Hello list,

I am using Thunderbird version 2.0.0.6 (20071022) with feisty and have noticed that even if the mailbox if empty in thunderbird, the
corresponding file in the filesystem ( in my case .mozilla-thunderbird/i8oq62wj.default/Mail/pop.gmail.com/) is not. For example the draft folder is 
empty but there is 3.8M of leftover:

-rw------- 1 yvan yvan 4012553 2007-11-15 16:10 .mozilla-thunderbird/i8oq62wj.default/Mail/pop.gmail.com/Drafts
-rw-r--r-- 1 yvan yvan    2258 2007-11-19 20:38 .mozilla-thunderbird/i8oq62wj.default/Mail/pop.gmail.com/Drafts.msf

How can I correctly clean Thunderbird? is it a rights problem?

Thanks for your help, cheers,

yvan