I wouldn't ever mock anyone doing exercise whatever their size, and realising that it takes much more effort both physically and mentally to do so.
I go to the gym in winter, often I'll watch the staff showing a fat person around, especially in the New Year. Often they do not join because they are intimidated by other gym users who they believe to be mocking them. The opposite is true! We all know walking through the door is the first and hardest step.