Quote from Space Pirate Nithiel;1202487:
Hundreds of years ago only the rich were fat. If you were fat it meant you had a ton of money and could actually provide for your family, while all the peasants living off scraps were skin and bones. I've heard some theories reasoning that some people get fat because of those leftover instincts from back then that you need to eat as much as possible when you can because you never know when you might not have access to food. Of course in today's society (most)everyone has access to food on a regular basis, making this unnecessary and resulting in weight gain.
The rich being fat is a stereotyped trope that passed down. It wasn't that rich people were fat, but rather poor people couldn't get fat.
To really understand this, you have to look at the amount of physical exertion the average person dealt with on a daily basis for work vs the amount of food they ate in general. The most fatty foods, like meat, was considered an extremely rare commodity that only the rich could taste. Most peasants would live their entire lives wondering what beef tastes like. Their diets consisted of whatever their country grew the most of; Asians ate mostly rice and cabbages, the luxurious ones could sometimes get fish, the Irish ate potatoes... or not. During the great potato famine, a third of the Irish population died. Simply because they couldn't properly distribute their potatoes.
You'll find that in poorer countries or cultures today, a large portion of their cuisine is made of soup. These soups are simply made with one or two anchovies boiled in water and cheap vegetables like cabbage. Other ingredients go in to make them fancy, but the overall concept seem to be similar and repetitive across the board.
It's pretty much impossible to get fat when you're running on the bare minimal amount of food needed to function. It's always a distribution and efficiency issue, rather than availability. I'd go as far as to say that is still the case today, with how much waste and excess food first world countries have. There are large amounts of crops going to waste to poor distribution or farmer patent wars.
Wait, I could've just pointed to 3rd world countries and made my point without referencing the past.
tl;dr being poor in a first world country doesn't even begin to compare with the true terror of being middle class in a 3rd world country.