Our SQL Server database for QC is getting pretty big. The QC database is now around 30GB... I did a short analysis and noticed that eg. the STEP database contains 72mil rows... Similar numbers for AUDIT.
It's filled with loads of (execution) data that's already years old. Given the fact that we run big testsets automatically multiple times a day, data management will become more and more important.
Since you mentioned running testsets automatically within QC I thought I would mention that QTP test results have multiple folders and files for each run. QTP comes with a Test Results Deletion Tool. Find it in Start>Programs>QTP>Tools folder. I scheduled the tool to run as a task to delete results automatically. See the Help doc within the tool for options.
QC 11 is supposed to have a better storage structure so identical files and not stored several times but linked too.