One of my most popular columns when I wrote for The Complete Running Network was on how frequently marathoners gain weight, rather than lose it, during training. To date, it's garnered more than 50 comments , which is a lot for that site and especially when you consider it was written years ago and still gets feedback from fellow souls.
I thought I might again look into why some of us bulk up when we are in the throes of heavy training. In that piece, a then-new study showed we eat more when we exercise and tend to justify the extra calories as an offset of those expended earlier in the day for a net-zero gain. But it doesn't balance out. We also tend to indulge in foods that satiate (pastas, breads), rather than those that leave us hungry within a couple of hours (like many raw fruits and vegetables).
Anyone who has kept a food and calorie log quickly realizes that the serving sizes on a lot of packaged foods and prepared meals are completely unrealistic for a typical appetite. Here is a 5-minute video (sorry, can't embed it) that explains why it is that the government sets the [granola] bar so low.
New York Times: Serving Size Snafu
Pay close attention to the data the FDA used to determine portion sizes. Then consider your own eating habits and how often you underrepresent how much you eat. If you are exercising heavily and still not losing weight, you may be chewing more than you bite off.