I'm not sure this is the case, although I used to think so. It's certainly true that hunter gatherer groups tend to get very obese on the modern, western diet. The Native Americans are a great example. I see Central Americans who come up here who get very heavy. On the other hand, the South Asians, who have had agriculture for a long time, also seem to have an issue with obesity, or at least they do when they start consuming the western diet, as is the case for East Asians.
However, even NW Europeans are about 35-40% Natufian according to Kurd's latest analysis. You'd think that would be enough "farmer" genes to take care of it.
Then, looking at the statistics for obesity over time in Europe, it almost seems as if the people in the area running from Italy north through Switzerland, Austria, the Low Countries, and Scandinavia hold the line pretty well, while the British and the North East Europeans to either side seem to have more issues with weight.
That means it can't be the cold weather factor, although I also considered that to be a possibility.
I think diet is definitely a factor, and I do agree that there is a big genetic component to this, but I'm just not sure it's just farmer vs non farmer genes. Unless it's a question of "certain" specific farmer genes.
@Davef,
No, it's not ok for a man to have a paunch, especially not a young one. Stop eating so much junk food! I'm sure your mother tells you the same.

Oh, and I was at my top weight of 140 when I was nine months pregnant! I was carrying another human being and other assorted bits. The doctor seemed to feel that was indeed healthy given that fact, but I couldn't wait to take off those 20-25 pounds. Fortunately, nature invented nursing.