I had already read articles showing that, while diet and exercise are most important (in that order), other things also have an effect on weight gain. They include lack of sleep (the next most important), temperature (apparently shivering burns calories), stress and medication/chemicals. But last year's New Scientist that I just borrowed from the library (December 2011, No2844/2845 Australia) had a couple I hadn't heard before.
One is pollution. Rats had 50% more abdominal fat after 10 weeks of being subjected to polluted air than rats on the same diet living in clean air. It can also "make your cells less sensitive to insulin, increasing your risk of type 2 diabetes." (p64) The link for diabetes has also been shown in humans. Nasty.
More startling is that the common cold virus, or at least one incarnation of it, has a strong link to obesity. Adenovirus-36 (Ad-36) increases both the number of fat cells and their size, and the effect lasts for years. One study claims "children with Ad-36 antibodies weighed an average of 23 kilograms more than children without them." (p63) And another 10 microbes have a fat-increasing effect. One researcher hoped that this means we can do something about obesity.
Maybe I don't have big bones. Maybe I have the fat virus.