header_cache database always growing

[Nicolas KOWALSKI <niko@xxxxxxxxxxxxxxxxx>]

Hello,

I am using the header_cache feature, for speeding up access to my imap 
folders. I defined it in my ~/.muttrc with:

set header_cache=~/.hcache

The ~/.hcache is a directory.

I have noticed that the headers databases are always growing, even if I 
delete mails. 

For example, my INBOX currently has only ~230 messages, but the headers 
database is really huge (INBOX was big, around 7000 mails, but I cleaned 
it a few weeks ago):

$ ls -l .hcache/imap\:niko@xxxxxxxxxxxxxxxxx/INBOX.hcache
-rw------- 1 niko niko 6463484 2007-11-20 20:10 
.hcache/imap:niko@xxxxxxxxxxxxxxxxx/INBOX.hcache

The database is "GNU dbm 1.x or ndbm database, little endian", as told 
by "file".

Am I missing something in the docs ?

Thanks,
-- 
Nicolas