I haven't tried another IMAP client.
I've search on Google and I've found this (this is the behaviour in my case, I've tried to change manually the maildirsize file and this is what happens)
>> > In an effort to increase the quota to 3GB, the size is being reset to less
>> > than 2 GB. Even hand changing the maildirsize file to the desired size, it
>> > will be reset by Courier later on to < 2GB value.
>> > After some investigation, it may be due to the integer size limit. This
>> > quota thing affects the maildirmake -q, and userdb.
>> > Is there any chances of Courier to support a larger quota size?
>> Not on 32-bit machines.