According to the Wikipedia, V-22 Osprey can lift 20,000lb of internal cargo, whereas it is able to carry only 15,000lb externally by dual hook. I wonder why there is a difference between external and internal cargo capacity. Is it due to the aerodynamics effect of the cargo?
The difference is because when the cargo is inside the aircraft the weight is spread across the cargo compartment floor, whereas when it is carried externally, the weight is concentrated entirely on one or both cargo hooks. The cargo hook attachment points are reinforced, but they still have a weight limit. When the cargo is inside the aircraft the weight limit is the MTOW of the aircraft.