Sure you can. You are stating there is some correlation between bodyweight and having a "scrawny" lower body. This connection does not exist. Many bantamweights have great legs. If your legs get bigger and stronger with bodyweight, so should your upper body. But your pull up ability should not change. Here's generally considered the strongest man of all time, Ed Coan, "finishing" his workout with effortless chin ups which he says he usually does "weighted". It's clear that exercise is not enough resistance for him. Go to 5:37. He had a near 1000lb squat.
No, you're incorrect. A guy who weighs 300 is pulling all that weight. Has to be much stronger.
A guy who weighs 150 or less is pulling that much less. Hang 150 lbs. of plates on him and see how he does.
The strength to weight ratio is totally different.
A guy with scrawny legs has an even higher strength to weight ratio for doing pullups.
A guy with no legs won an NCAA wrestling title. Why? He had the upper body and strength of a guy 50-60 lbs heavier.