Bonus: Does the V-22 get some of its thrust from the exhaust?
The rotor size helicopter (or a tiltrotor, in this case) is determined by its aerodynamic requirements, most importantly, its hover characteristics. The main constraint is the weight that has to be lifted by the helicopter at a give a altitude.
In this context, it can be seen that the blades of V-22 are actually smaller. For example, V22 has a Max. TOW(Take-off Weight) of 27,400 kg. For achieving this, the V22 twin rotors of 11.6 m diameter, giving a rotor disc area of 212 m². A CH47, which has a Max TOW of 22,680 kg has a rotor disc area of 520 m² using twin rotors.
If we take single rotor helicopters, the CH 53 Super stallion has a 24 m dia. rotor having a disc area of 460 m² to lift a max TOW of 33,300 kg.
This lesser blade disc area of V22 limits is ability to hover at higher altitudes. The V22 rotor blades will have been optimized for required performance parameters. So, if the blade length is reduced (assuming airfoil is kept the same), vertical takeoff is possible though other performance characteristics will be severely affected.
The V 22 uses two Rolls-Royce Allison T406/AE 1107C-Liberty turboshafts, which do not produce any thrust.