SK, I am 6'3", weigh 171 lbs, and bench 145 for 10 reps. Nothing to brag about (at least in my book).
I have a younger brother, who is 6'2" and weighs almost 180 lbs. He cant bp 120 lbs for 3 reps. When i started out in January, I was benchin 95 lbs for 10 reps.
Now if you saw my bro and I, you would think we were twins. I dont think genetics plays a role in this scenerio. If you look at his chest and tri's compared to mine, you would see why. If you look at his bi's compared to mine, his are much larger. DId I mention that I would guess his bf to be around 4-5% and mine is between 10-13%?
I dont think you can compare your very first lift to someone of the same height and weight that has lifted for a good while. There are too many other factors like spats has mentioned like recruitment. A muscle that is never used, might have size, but it is nothing compared to someone that actually uses that muscle.
On the other hand, you are right, according to physics, the shorter arm has the advantage, but its only one factor of many.