Seems to be the upshot of this finding, I'll Have What She's Having: Effects of Social Influence and Body Type on the Food Choices of Others:
This research examines how the body type of consumers affects the food consumption of other consumers around them. We find that consumers anchor on the quantities others around them select but that these portions are adjusted according to the body type of the other consumer. We find that people choose a larger portion following another consumer who first selects a large quantity but that this portion is significantly smaller if the other is obese than if she is thin. We also find that the adjustment is more pronounced for consumers who are low in appearance selfâesteem and that it is attenuated under cognitive load.
The influence of peer groups on how you eat, and what you eat, and how much you eat, is obviously an important issue to be investigated. Too much of the discussion of food focuses on individual actions abstracted from ecological context. Environmental cues no doubt reshape the way one implements heuristics which allow for choice, even if there is also an innate individual disposition.
Makes sense -- if you see a fat person nearby, you are more likely to be conscious of portion size. But a thin person doesn't set off any mental alarms (I suppose it might even set off the opposite sort of reaction if the thin person was not merely slender but looking malnourished.