Not sure what you mean by number of rows increased, a RESIDENT load will take, if there is no WHERE clause of any type, all the rows from the previous table.
Do the Base and MDTB have the same fields? If so, the RESIDENT load is incrementing the rows in the MDTB table, and the Base table will be automatically concatenated to the previous one (MDTB). Maybe that's why the number increases?
Also, if you are using RecNo() without "AS" in any other table, which is technically possible, you are associating those two tables.