That bothered me when I was at my height of fatness, but now I couldn't care less what some morons say. The fact is the most people out on the trails (running or biking) are more impressed with a chubby guy like me getting out there every day and busting my ass than they are with the morons who yell things at me about being fat. I've noticed I get more "thumbs up" and "good work" from others than most of the fit folks do.
I refuse to let a few narrow minded morons who have their own self esteem issues to bring me down.
That said, the only motivation I need is the cute girls pedalling by me on the paths. IDK why but when a hot babe passes me on a bike I suddenly can pedal faster. I'm married but not dead. lol.