By default deleted customer records are kept in the client.dat and the file continues to grow. This is a problem for large clients since 2gb is the file size limit.
To have Retail Pro discard all the deleted records and compile a new file with just active records, do the following:
Select REPAIR and open the client.dat file
EDIT any field on a deleted record
Select FILE > SAVE and then select NO to have Retail Pro repair the blank free space records.
The new client.dat should be much smaller now.