One of the most insightful studies I've read on dieting is that people trying to lose weight underestimate how many calories are in the food they're eating. It's not surprising once you start logging everything you eat -- you'll see that a lot of common foods are extremely calorific.

I can't find the exact study but here are two on the matter

http://www.ncbi.nlm.nih.gov/pubmed/16954358

http://www.bmj.com/content/346/bmj.f2907