A bit of everything....
First shot is the before shot, which is the day, autumn, camera in the following.
Lighting ....day vs night shot, autumn vs winter shot, and camera vs webcam shot.
Next up is my diet.
First shot far less than 4% body fat, and barely eating (coffee, cigarettes, and a bit of food).
Second shot other than gym, around 4% body fat (still low, but working on it with my metabolism) eating like a pig not only due to gym, but also cut down cigarettes (tomorrow is the first day of no cigarettes).
Calorie intake has never been a problem for me, and I still don't drink enough water (but I try), but to put it in perspective, I'm 181cm tall, and in the first photo I weigh 59.5KG (131pounds), and in the second photo 68.5KG (151pounds).
Edit.... "mean tough-guy scowl" is my bad effort at trying to pose for a photo