Most thin/healthy/average people assume that heavier/fat/obese people move heavily, are slow, and cannot be spry. When, in reality, there are heavy people out there who weren't always heavy, whose bodies still move like they did when they were lighter, excepting the extra bulk.

In my personal experience, my aunt is morbidly obese (she's working to lose the extra weight, but it's there for now) and she moves like an obese person. She needs help getting into and out of most seats, she moves slowly, and she lumbers up and down stairs. In contrast, an old supervisor of mine was equally obese, but she flitted about the office, skipped sometimes, and in general acted like she was a girl 1/3 of her size.

So, in answer to your question, I think RJ and Sanderson for that matter, are giving into the general social prejudice that obese people will move heavily in general, but they are allowing their obese/fat characters to be exceptions as noted by the main characters.