Alas, the "less calories, more exercise" approach, while it works for many, is a little simplistic.
On the eating side of the equation there have been a lot of studies that show it's not just how much you eat, but also what you eat. For example, in studies where two groups got exactly the same number of calories with one group getting more calcium (from dairy) in the other, the group getting more calcium tends to maintain or loose weight when compared to the other group regardless of activity levels. Other studies have shown that people who tend toward foods with a lower glycemic index, meaning they have fewer processed sugars and starches and take longer to digest so their calories hit the bloodstream more gradually over a longer period of time, are less likely to be overweight than people who are just as active but consume more processed sugars and starches.
Then there's also the case of consuming too few a calories, giving your body the message that it has to conserve energy. People who consume half or less than the suggested number of calories for their body size, build, and activity level loose weight substantially slower than people who consume more food. And just to make things even more difficult, different people's bodies start trying to conserve calories at different dietary levels.
So it's not just about the calories. It's about eating right and about eating the right amounts for how your body works.