Yeah I agree that what you eat is a huge problem. But that means it's not just about calories in, calories out. Some posters here seem to think that is an absolute, and he specifically targets that mentality (as well as the idea that all obese people are just weak willed sloths).

He also mentions at the start of the video I link below that "obesity is part of the human condition. There are about 60 different medical causes of obesity." He doesn't go into it obviously as the video is about the current obesity epidemic, but the point remains that there are other factors contributing to obesity, that can be more important than calories in/out. For example, It's fairly well accepted that stress for example will make you gain weight easier, and smokers will gain weight if they quit.

http://www.youtube.com/watch?v=0z5X0i92OZQ
(It's the same guy delivering another version of the same lecture, slightly different anecdotes/slides in parts.)

The video caption is
"In a special event hosted by the County Office of Education on March 24, 2011, Dr. Robert Lustig, Professor of Clinical Pediatrics, in the Division of Endocrinology Director of the Weight Assessment for Teen and Child Health (WATCH) Program at UCSF, speaks about the role of sugar and the contribution of biochemical, neural, hormonal, and genetic influences in the expression of the current obesity epidemic both in children and adults."