You seem to have missed my operative point: I’m not talking about in-person, real-life comments. I’m talking about my OP. Others have taken up the thread about direct comments.
Agree. Skinny does not mean healthy or “fit”. It doesn’t mean fast either! I think it’s different mechanic at play though; not self-conscious, per the OP.
…skinny people can be self-conscious too - when I first started running I’d only run after dark and away from the main path because I was so out-of-shape and I felt like a jackass. It’s too easy to get discouraged, no matter what your weight is.
But yes, I can how bigger people would be more affected by self-consciousness.
I just wanted to share a story about this topic. When I was a young kid, I saw a very overweight person riding a bicycle down the street. I started to laugh and make fun of him, and my mom very politely pointed out that we shouldn’t be rude, because he is actually trying to be active and get into better shape. I will never forget that moment because I realized how stupid I was for laughing, thinking an overweight person had no business riding a bicycle, only to realize thanks to my mom that we should be advocating and celebrating exercise. Ever since then, whenever I see someone working out, I think “good for you!” regardless of whether or not they are overweight or in great shape.
Aah, I didn’t read your post closely enough, apologies.
Although it seems like you got quite a bit of agreement in this thread and are focussing on two vocal naysayers. This is the Straight Dope, you should feel pleased to get any agreement! 
Yep. I should have put this in the Pit, and somehow included mention of both rape and people in wheelchairs. THAT would have made for a much more Dope-typical discussion.
I was excercising on the stationary bike in my basement 3-4 years ago when a roommate walked in and remarked how the little seat dissapeared under my fat ass. It takes a special kind of jackass to call someone fat WHILE EXCERCISING… it’s pretty funny now though.