No. There were fat people in the middle ages. There are a lot of fat people in Africa - only because all the media shows are starving children doesn't mean there are no fat people in Africa, just google for it. In some countries it is even worse than in America for example. And last time I checked Skyrim was attacked by dragons and not suffering from a hunger catastrophe.
If you look at the ES world it is a pretty nice place actually. People live in harmony and in nice houses, they have food lying around everywhere. They even store some of it in crates and barrels outside their homes (apparently because there is no space left inside their houses for more food n drinks). Shops sell all kinds of food, every tavern has a nearly endless amount of it. There are rich and middle class people wearing expensive clothes. More than enough potential candidates for becoming fat. Not everyone in Skyrim will be a tough hunter living a harsh life out in the woods.
I don't really mind that there are no fat people, but 'explanations' like these drive me mad. If there were no children in Skyrim I bet someone would give a serious explanation why it's totally realistic to have no children.
I don't mind either, but excuses like this, which completely ignore the bulk of the explanation given, drive me mad. The size of wealth doesn't matter much, and if you read the explanation you'd see that it wasn't even mentioned. The mountain environment conditions themselves don't easily permit there to be people with excess baggage. I'm just going to quote myself:
they wear heavy layers of furs to keep warm (mean lots of burnt energy) and engage in physical movement to raise their body temperature (also means lots of burnt energy).
On top of that, in Skyrim they're at a high elevation, which means thinner air, which means every action takes a lot more energy than normal and the body gets fatigued a lot faster than normal, which means it will rapidly burn off fat. And, considering the dynamic terrain, anyone who moves anywhere outside their own house is going to get a workout. They also don't eat fast food and the other garbage that contributes to obesity in today's world, so their nutritional habits themselves create a leaner populace. And, they don't sit around in front of computers and desks all day, there is always maintenance to be done to continue their mode of life, just as their is today by people who live within similar means. In those conditions, you live to work to live.
And fat people in the middle ages owned their own estates with servants enough that they had to do nothing. A single Skyrim town would be the number of people that would serve one such a fat person, ignoring the effects of the mountainous Skyrim environment.
Again, it'd be heavily unrealistic to have fat people in Skyrim.