The age old saying that 'the camera adds 10 pounds' for the most part is true, but it's not like you gain and lose 10 pounds every time someone snaps a photo of you, so what's the deal?.
Firstly there's lighting to consider. Flat and even illumination given off from a camera flash or bright lights leaves little room for shadows, an important part to adding depth and dimension to a persons face. Secondly there's focal lengths, the distance between the camera lens and the camera sensor.
In a recent experiment photographer Dan Vojtech decided to experiment with varies focal lengths and the effect they have on a portrait photo. Using himself as the model the results are surprising to say the least with the difference between 20mm and 200mm focal length clear to see. They say the camera adds 10 pounds and judging by this it's true.