Online defrag priv.edb does not run!

I'm in charge of an Exchange 5.5 server of which the priv.edb has grown to a 35GB+ size. After checking the configuration i noticed the the retention time was set at 7 days with a setting of don't permanently delete items until the store has been backup up. I changed the 7 days in 0 days and cleared the checkbox. Expected to free up approx about 18GB(items in retention) in the priv.edb, but for some reason the online defrag now only runs for the dir.edb and pub.edb. Modified the ArcServe settings not to run for 2 days, as not to interfere with the scheduled defrag but defrag still doesn't start. I can stop the exchange services to run an offline defrag, but the server is nearly out of harddisk space and the last time the services were halted the priv.edb for some reason added 6GB to its size after restarting the services. Obviously before doing that again i'd like to know first how much space the online defrag will free up, before running the offline defrag.
Please help, can't seem to find an answer anywhere!