First, in reference to what some people are calling a "hit bubble", HiTech has said on several occasions that the hit bubble is the shape of the aircraft, period.
Second, with regard to multi-engine fighters being more prone to pilot wounds, I think several of you are not taking into account the configuration of the aircraft.
Consider a P-38 diving on a base vs. a P-51. The P-51 pilot has a large engine block in front of him. Hits by small-caliber rounds impacting the front or underside of the front of the aircraft hit this engine block, possibly doing no appreciable damage to it, and certainly do not reach the pilot. In the P-38, no engine block to shield the pilot. Small caliber rounds are therefore more likely to pierce the thin skin of the aircraft and impact the pilot's compartment.
Now consider those same planes after the dive and on pull-out. Fire coming from behind the P-51 pilot has the rudder, elevators, vertical and horizontal stabilizers, and a good portion of fuselage that is in the way, and can all be hit / damaged before reaching the pilot. The P-38 pilot's compartment by comparison is very exposed.
A legit question IMO is whether the pilot's compartment is the target "object" that needs to be hit for there to be a pilot wound, or is the pilot himself modeled into the aircraft as a separate hit object. My belief is that the pilot is modeled, but I do not recall having heard a definitive answer on that subject. If it is not, then any aircraft with a relatively large-volume pilot compartment (read - Mossie) would be at a disadvantage.
Finally, I am a bit perplexed at the claims that the P-38 isn't that much bigger than a P-51.
LOCKHEED P-38J LIGHTNING:
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spec metric english
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wingspan 15.85 meters 52 feet
wing area 30.42 sq_meters 327.5 sq_feet
length 11.53 meters 37 feet 10 inches
height 2.99 meters 9 feet 10 inches
NORTH AMERICAN P-51D (MUSTANG IV):
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spec metric english
_____________________ _________________ _______________________
wingspan 11.28 meters 37 feet
wing area 233.2sq ft
length 9.83 meters 32 feet 3 inches
height 4.16 meters 13 feet 8 inches
The P-38 is about 40% larger wing span and wing area -- that alone says it should be hit much more often than a Pony given any standard dispersion of ack fire.