cr0w
Super Freak
Your body rebounds extremely fast after fasting like he did for machinist. A lot of that weight was glycogen stores in his muscles and water retention from adding electrolytes back into his diet. It isn't anything like adding 80 lbs to someone who is at their natural weight.
Bingo.
Also, a good portion of the new suit in Man of Steel is sculpted muscles. Cavill isn't actually as big as the suit makes him look. Similar to how they did with Routh's shoulders and pretty much everything on Tobey Maguire's Spider-Man suit, just on a larger scale because he's, you know, Superman.