When you passed him, did you make any eye contact with him? Maybe if you looked at him, he took it as condescension. Perhaps he thought you were looking down your nose at him. You did mention, "good for him for getting out and getting some exercise!". As innocent as that comment is, it does have a wee smell of superiority to it. Like, "look at the fat guy on his mountain bike on the sidewalk...good for him for putting an effort into getting some exercise!" Maybe he was just getting from point A to point B by the only means he can afford.

BTW, I like your use of the word rotund.