could be two reasons....
1) most corporate gyms dont like bodybuilder type people in their gym as it scares most female gym members away. They want to feel comfortable in the gym they go too..hence, the women only gyms... Some gyms in my area do not even allow deadlifts or squats, they say its for insurance...I call BS.
2) If your a wanna be, I would give you a dirty look too. I love watching the 150lb guys doing 1/4 reps, cheating the whole way... or doing that massive 225lb bench and taking it down 2 inches. Or doing SQU's....hence its not a full squat, so my crew started calling them SQU's....or my all time fave, the curls where the elbows never get unbent past 90 degree's, just more like bending over and bending back up....
.... Not saying YOU train like that, but if you do..its not really a dirty look I would give you, but more of like a "your a f'ing joke dude" kinda look. Because in my eyes if you train like that, your just wasting space and taking up gym equipment I could be using.
If your a serious BB'r and train correctly with proper form... then its probably issue number 1. I don't know how many whispers i get behind my back only hearing a couple words.. but you get the gist of it too when you hear something like "well if I took what he does, I would look like that too".. Can't say they are talking about me, but I hear shit like that all the time from comming from somewhere behind me... I mean I don't think of myself as a "big dude"..I am far from cutler or ronnie... hell i would even call myself soft right now.... 275ish (depending on water that day)... bodyfat is probably in the 10-12 range as some abs are still visable... maybe I just have a distorted self image... I was a chubby kid, so no matter how I look I always feel chubby...
As long as you train with proper form and dont try to use weight you can't handle properly, screw everyone else and what they think or say!! Don't give them a reason to talk about you by doing pull downs and using your entire body to pull it down. (not saying you do this, but you know the type)