The pilot vessel used by Balok was a small ship capable of enormous power. It controlled the flagship Fesarius and was equipped with a tractor beam. Other than Balok himself, no crew was necessary.
On stardate 1514.1, the USS Enterprise was forcefully towed by the vessel on a heading to Carpi and could only free itself under great risks. (TOS: "The Corbomite Maneuver")
The script of "The Corbomite Maneuver" made it clear that Balok's pilot vessel was a section of the Fesarius which detached from that mother ship.