Ads don't always (and you could argue rarely) show their target demographic.
I see plenty of overweight people in ads for diabetes treatments. I see plenty of fit people in ads for workout machines. Right now I see an ad on the side of a bus encouraging people to get free STD testing, and it shows people dancing at a club.
There are aspirational ads, but the use of "rare" in your comment doesn't reflect reality.
The era of 'supersize me' fast food marketing is long gone. Most fast food restaurants are not targeting that specific of a market anymore, they're aiming at a much large cross-section of the population. A better example would be an all-you-can-eat buffet.
According to the CDC, over 70% of American adults are overweight or obese, so by targeting a "large cross-section" of the population restaurants are in fact aiming at 'large' customers. This reality isn't represented in advertising for obvious reasons.
Specifically targeting a population, and the coincidental demographics of that population are two entirely different things.
That 70% is part of the 100% which they are actually targeting, but only coincidentally. There are plenty of fast food ads targeting health-conscious consumers.
You could also point out that 70% of Americans are also Christian, but that doesn't mean every company that sells to Americans specifically targets Christian customers...
...also, most large fast food companies also have a significant presence in multiple countries.
They'll usually use slightly older kids in a toy advertisement than the actual target demographic they're trying to sell to.
Part of that is that kids generally don't want to be seen as playing with toys for younger kids.