Mover List Parameter is only displaying 25 out of 300 possible customers

I have a report which contains a Parameter that is set to allow Multiple Values, which will display a mover list for the users to select from. Th issue is the fact that I know the database has 300+ possible customers, but the parameter is only making 25 of those 300 customer records available to select from. How do you get the Multiple Record Parameters to display ALL available records? why is there this type of limitation?