Except that the variations of the gene associated with obesity are also associated with eating a huge amount more:

http://onlinelibrary.wiley.com/doi/10.1038/oby.2008.318/abst...

Which is a much simpler explanation.

And even so, carriers of even the highest risk form of the gene are still on average only 6.6lbs heavier than normal:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2646098/