Why are people fat?

I see a lot of speculation and research on why people are fat--everything from fast food, to TV, to women returning to the workforce in the 1970s, to antibiotics in childhood, to genetics. So when I look at pictures of paintings from the 17th century, or photos of my family reunions in the 1920s, and no one had fast food, everyone had physical work, there was no modified or processed food and they look just as overweight in middle age as today, I do wonder what the researchers are missing. Frick Hals portrait