Hi George,
I'm assuming your in SQL. Which version?
First step, identify where they have the more data. Do they have a huges number of inventory? Lots of transaction in the sales side? in the purchases side? Serial number maybe?
The idea is to find which tables are too big, once you find them, check out the indexes, ASC copied all their lan indexes to their sql version but in sql sometime you can remove some of these index without affecting the performance. Also, sometime you'll find indexes with fields your client don't use like those new inventory special fields they added in version sql5.05, removing those index helps reduce the database size.
Let me know your finding and I'll help you more if I can.
HTH
Diane Robitaille eng.