Since they are in use by the application and SQL server, there are no options to reducing these file size (these files can grow.) However there is a need maintain the database in good health and available resource to support the application, to that end, please see the article about Database Maintenance to include information on Disk optimization and the need to increase available disk space.
Allen Brown from ProjectWise development has pointed out that in his blog “ProjectWise Performance Tuning: Part 1” that the key thing to keep in mind with SQL Server is the more disk spindles the faster access you have to the data. So bigger isn't always better if that big database drive you have takes forever to retrieve the data. Since SQL Server may be writing to the log files and the database files it is very import to keep these files on separate drives as well.
• User Database files (.mdf) should reside on a separate drive from the data files for the System Databases.
• Log files (.ldf) should reside on another drive from the database files (.mdf), but can reside with the other log files.
http://communities.bentley.com/products/licensing/w/licensing__wiki/database-maintenance.aspx
