Exchange 2000 mailbox size discrepancies

Running native multi-server EE Exchange 2000 SP3 w/ Outlook 2000 clients. When a client reaches a standard mailbox limit Exchange sends out a system message warning that the client has reached its mailbox limit, lets say 50MB/mailbox. When you compare the exchange limit to the actual Outlook mailbox size they are quite different. 25MB vs 50MB limit? Any ideas on what could be causing this and how to correct it. Thanks.