"A given person, in 2006, eating the same amount of calories, taking in the same quantities of macronutrients like protein and fat, and exercising the same amount..."

"... as a person of the same age did in 1988 would have a BMI that was about 2.3 points higher. In other words, people today are about 10 percent heavier than people were in the 1980s, even if they follow the exact same diet and exercise plans. 'Our study results suggest that if you are 25, you’d have to eat even less and exercise more than those older, to prevent gaining weight,' Jennifer Kuk, a professor of kinesiology and health science at Toronto’s York University, said in a statement. 'However, it also indicates there may be other specific changes contributing to the rise in obesity beyond just diet and exercise.'"

From an article in The Atlantic called "Why It Was Easier to Be Skinny in the 1980s."

If I had to guess what's going on here, I'd guess that there is inaccuracy in estimating the calories consumed and the weights of the individuals in question, especially those people from 30 years ago. But Kuk's guesses are: 1. exposure to chemicals in the environment, 2. prescription drugs, and 3. our "microbiomes" — gut bacteria — have changed.