For example, compresses a message with a length of 50000 down to a length of 35000, or some such amount. Then the compressed message is stored in the database, then the message is decompressed to it's original length each time it is selected out of the database by a user.
Never heard of such a mod. It would also mean that search.asp would have to be dealt with, because the text would also need decompression before being searched. If you think about searching, doesn't look like a good thing to do. And DB space is not such a big issue today, so I guess I would skip the compression, unless it was done transparently by the DBMS itself.