Hi BrianPoole,
There is a limit of 1million cells of data being retrieved via the api. So if for example you specify the fields something like, RecordID, Name, BillAddress1, BillAddress2, and BillAddress3....that's 5 fields. 1 million/5 = 200.000 records at most being returned.
It sounds like in your case you will need to specify a range of records using the where clause.
In the return xml there is an atribute on the MethodAPI element, MaxRecords. When this is true it means there were potentially more records that could have been returned but were not, due to the million cell limit being reached
e.g. <MethodAPI response = "Success" MaxRecords= "True">
Dave