[fatdog789]

The consensus of that video was that calories eaten vs. used/expelled is the primary factor behind weight gain, but that some types of foods contribute more to weight gain than others for various reasons.

For example, fats are more easily passed through the digestive system (in the sense that they often just flow through to the other side), while sugars and starches are more likely to remain in the system and actually get digested. (Incidentally, the body's inability to fully digest fats is the way Alli works -- it makes your body even less capable of digesting fats, so if you eat too much fat, it comes out your ass.)