We're testing the Nokia 7650 with a SyncML server that we're dveloping. It still has some bugs that's why synchronization is not always successful.
We noticed that every sync attempt that we make, the space occupied by the address book is increasing, even if the sync was unsuccessful. Do take note, the account on the server does not have any data yet, so the phone is the only one sending the data on the addressbook, the server is not sending any, only device info and the status for each command sent. This happened over and over again until there was no more space on the 7650. This happened to the point that we can't even turn on the phone anymore and have to send it over to the service center to re-flash it.
I don't know if this is the cause, but the contact database at any Symbian OS Nokia phone will be compacted every now and then. The database size will inrease when user or software reads/modify it, but the phone should compact it again.