I am not aware of any circumstances on a mainframe where the situation you describe makes sense or returns a valid count.
Fixed length records are that length, period. Nothing is added to them. If the LRECL is 40, the correct byte count will be 40 times the number of records. If the LRECL is 85, the correct byte count will be 85 times the number of records. If the LRECL is 32760, the correct byte count is 32760 times the number of records.
Variable length files have an additional 4-byte field at the beginning of each block (called the block descriptor word) and a 4-byte record descriptor word at the beginning of each record but not two bytes. Of course, with variable length records you have to sum the record lengths to get the correct byte count.