I'm practically sure that you're not actually talking about Quota.
The quota is set by the administrator, and there is no way you can diminish it on the client side.
On the other hand, the "% used" in the database properties box is indeed something you can act on - but whether to do so or not is another question.
The % used number merely indicates what portion of your mail file is used by messages. There is nothing that mandates you keep it under 80%, or whatever. It is an indicator, that is all.
Pascal.
I've got nothing to hide, and I'd very much like to keep that away from prying eyes.