Database shrinking was rather usefull for some old SEPM 11.0 releases. If you want to reduce the space consumed by a SEPM 12.1 database, I would suggest you to:
- Reduce the number of definitions you keep (http://www.symantec.com/docs/TECH92225)
- Decrease log retention (Admin > Servers > Database under Local Site > Edit Database Properties > Log settings)
- Increase Transaction log truncation frequency (http://www.symantec.com/docs/TECH166658)
- Delete old client packages from the console (Admin > Install Pacakges)
NOTE: keep in mind there is a defect regarding Transaction log not shrinking on SEP 12.1 Small Business Edition. This is fixed in later release (http://www.symantec.com/docs/TECH187656).
Here is an interesting article about SEPM sizing:
http://www.symantec.com/docs/TECH123242