What I think you are referring to is the option in Access 2007 called "Compact on Close" which will automatically compact your database whenever you close it – however you must open then close the mdb using Access itself; the option does not apply when other applications (such as Path) open and close your database.
Unfortunately whenever you manipulate an Access database it seems to grow exponentially in size – the same is true when Path reads and writes to the database. At present the only solution is to periodically open your Path mdb file in Access, select Manage from the Office button, then select Compact and Repair. This will typically reduce the size of the mdb considerably.
If your mdb file is still large you can consider deleting some of the run results. It is also easy to copy scenarios from one database to another, and to have multiple databases open at once, so another option is to move some scenarios to a second copy of the mdb.
Hope this helps, Colin.