For me. Arm pits first and foremost. I don't wear deodorant, in fact so my Natural scent and pheromones. That gets me horned on a guy easily. Second to that is crotch scent, washed and clean but the Natural male scent emanating from it as I go down on their hard cock.
I know what you mean
 
From my experience, and it's only what I've noticed, is that it's not so much the guy's muscle size, but whether the guy has sold muscle versus a soft body. And the more fat he has, the less that he smells like a guy with solid muscle. One can be skinny and soft, or he can be skinny with hard muscle, like a feather weight MMA fighter or wrestler, for example.

Ok, but one can just be skinny, and not have much in the way either of fat or muscle.