In a recent column at VoxEU.org Rachel Griffith and Melanie Lührmann argue that its because he doesn't exercise as much we used to.
Griffith and Lührmann start by arguing that the rise in obesity we see around the world has largely been attributed to an increase in calorie consumption. They then set out to investigate this claim by examining the evolving consumption and lifestyles of English households over the 30 year period between 1980 and 2013. While there has been an increase in calories from restaurants, fast food, soft drinks, and confectionery, there has been an overall decrease in total calories purchased. This decline in calories can be partially rationalised with weight gain by the decline in the strenuousness of work and daily life, and increasingly sedentary lifestyles.
Overall, we see that declines in physical activity at market work and in other activities has largely counteracted the reduction in calories. As well as going some way toward explaining the rise in obesity, our research indicates that market work might also influence the types of foods we eat. Households that spend more time in market work buy more market-produced foods. They eat more often in restaurants and fast food outlets, and eat more takeaway. Market-produced foods are on the whole more expensive than home-produced foods, and this means that trends in expenditure do not necessarily mirror trends in calories.So its not that we consume more calories, its more that we don't use up as many calories as we used to. Thus its not so much that Fat Bastard is tucking into more haggis, its more that he not chasing and cubing as many Englishmen as he used to.