(Thread Necromancy is why they call me "Darkfellow". . . Or maybe it's just 'cause that's the handle I chose! XD )
I don't find the attitude of targeting muscular guys to be highly prolific, but I have witnessed it on multiple occasions. There's definitely an issue here, and I find it's probably created by a few factors.
Firstly, that many people get into martial arts for bad reasons. I'm talking mostly about the same kinda' guy who becomes a cop because he wants the authority and power over others. These guys take up martial arts because they feel vulnerable, and to prove to themselves that they're not weak, they go after people who are perceived as being more powerful. Even if this guy is afraid to actually fight the muscular man, he can exhude an air of bravado to create an image of being not afraid of him, and call that guy out on being all muscle, no skill. Foolish and flawed, but it happens.
Then there's the guy who just makes too many assumptions. "Oh, that guy spent all his time lifting weights, so he never learned how to fight." Simple, and dangerously erroneous.
Simpler than that, is the guy who's just jealous and so tries to create a negative image around the object of his envy.
Then there are the guys who are similar to me (but possessed of more ego). Guys like me go into the gym and look for the biggest challenge, often biting off more than we can chew. We crave the competition of the fight, the beauty of the art, the spirit of the personal warfare, and/or the test of personal advancement, and these guys seem like prime targets. Especially if you're asking yourself, "Have I trained well enough to really make the art work?"
While I don't personally target these guys, I am always aware of those elements I just described whenever I happen to partner up with the muscle-man in the gym. I certainly want to beat him more than I care about beating the other guys, because of the ego-boost it would grant. Fortunately however, I have enough self-control and self-awareness that I don't let those emotions drive the way I practise. When I get paired up with the muscle-man, I still go at normal speed and intensity, I submit as soon as he catches me, I'm respectful in my language and action. I don't go easy on him nor expect him to go easy on me. We practise, eventually it's time to change partners, I thank him and move on. Exactly as I feel one should do, no matter who they're partnered with.
That said, I've been in a lot of gyms and talked to a lot of people about martial arts-- so I know first-hand that the "Get the big guy!" attitude definitely exists.