I thought back and it was true. Now granted, San Francisco is probably not unique--in all urban areas where one can walk a lot and have access to high quality food, obesity rates are likely lower. But I would posit that the added strain of the major hills in SF definitely contributes to a slimmer population :).
I was very proud of my mom, who is just over 60, and handled the hills like a champ, including Filbert Street, one of the steepest hills in the Northern Hemisphere. I think I've been blessed with a pretty darn good gene pool. Hardy farmer stock on one side, hardy Jewish peasant stock on the other. When my maternal grandparents passed away they were over 90, and my paternal grandparents are both alive, which is pretty amazing.
I love the phrase "genes load the gun, environment pulls the trigger." Walking is my favourite form of exercise and I usually live in very urban, easily walkable areas. I have also never been overweight (even though I ate a lot of crap in high school and the first part of college). Whatever genetic hand you've been dealt, environment has a huge amount to do with how your genetic code is expressed.
I still believe what and how much you eat is by far the biggest factor. But the difference between having to walk a mile to and from the grocery store every day and having to drive because there are no sidewalks where you live can make a big difference. My running joke is that I am in good shape now because I live on a hill and I have to go down and up it no matter what if I want food. Obviously, I could get take out every night (back to the what and how much part) or have my groceries delivered. No matter what, today one needs to make a conscious effort to get exercise. But certain environments absolutely make it easier. As does the fact that I am usually too cheap to use Peapod or get crappy takeout ;)