The Marines didin't get 13 men to fit into a UH-1Y, which is their notional infantry squad. (Won't digress into the doctrinal arguments between our Marines and Army on whay an 11 or 13 man squad ... )
I believe the USMCs prime troop mover is the CH-46 and recently its replacement, the MV-22 - two squads (three fireteams per squad).