PM summed it up,
but there is also the lack of promotion and advertisement for body care, gyms, sports and clubs.
And the stress free environment (yes stress is a calorie burner, I proved it :) )
Those who get fat here would have been just as fat in any other country, if they worked and worked out the same way they do here.
One final note, if ur not fat urself, u have no right to ask such question!