Unlike what most people think, Brad Pitt did not need to workout hard to look like he did for Snatch. Snatch was made within a year of the completion of Fight Club. Most of his physique is from maintenance type training. The focus of such a regime would be ensure no loss in muscle mass.
Brad Pitt's body fat percentage in Fight Club was roughly around 7%. By Snatch it was more like 9%. It is not healthy for someone to stay at such a low body fat percentage for long periods of time. He cut to get to the 'Fight Club Look' that was required for such a film. After the film, he relaxed his cardio and strict diet to ease himself into a more normal and manageable physique. So, what you see in Snatch are his heavy bulking efforts from Snatch combined with a less strict diet.