True, but if we're comparing a moped(about 203 lbs dry weight) and a starter motorcycle like a Honda Rebel(331 lbs curb weight), would that be a huge difference?
I can understand some differences with injury type, but what about fatality? Because I figure it's almost always going to be a head or neck injury that ends up killing you, and that likely won't change upon vehicle type unless your neck somehow got crushed under your bike. Now I suspect having a full face helmet could protect you better from projectiles(could just be a small rock), or having your face literally kissing the pavement in a spill, but I think hell would freeze over before you catch a cyclist with one of those

.
As far as impacts, I'm pretty sure they all tend to end up like this, even on an 800 lbs chopper:
[youtube]Had20POo6ZA[/youtube]
Btw, I suspect that was taken in Thailand(guy drove right through the red light).
The fundamental problem being that 2-wheelers don't have a "crushable" frame to absorb impact, so any additional impact that the extra weight can absorb is more or less a moot point.
My personal opinion is that fatality should be the same at all speeds, and depends on the helmet type. Injury could vary as you said due to weight, but I feel that's almost 100% dependent on your safety gear.
I would bet that a cyclist doing 30 mph with nothing but spandex and a helmet is going to end up with more expensive medical bills than someone in a motorcycle at the same speed with full gear(thick leathers, impact armor, full face helmet, full body coverage).
Again, my planned expenditures on the gear would probably be in the neighborhood of $2,500(Kevlar Mesh, with customer feedback rating it comfortable up to 85 degrees with liners, and 120 without. Today was a "hot" day at highs of 86). My attitude regarding traffic laws is to be a PITA walking around to the nearest crosswalk while my family jaywalks across a street with 2 lanes. My attitude towards other drivers when walking is to go around
behind a car that's halfway sitting in the crosswalk for a right turn, so I generally don't expect other people to actually follow the law, because I realize that words on some piece of legislation will not physically stop people from running me over with a 3000 lbs brick on wheels.