The actress - who has taken over the role of Mystique from Rebecca Romijn - told Parade that she felt it was important to maintain her curves to portray the character realistically.
"When I got the part [of Mystique] and knew I had to be naked, I didn't want to look like a pre-pubescent 13-year old boy. I wanted to look like a woman so I knew I wanted to maintain my curves," she explained.
"I'm so sick of seeing these skinny little women and thinking I'm supposed to look like that and thinking young girls think they should also look like that. But of course there was working out; I had a trainer and I had a nutritionist. I worked out for two hours a day for seven days a week."
Lawrence admitted that she was worried such an intensive exercise schedule would cause her to shed too much weight, adding: "I thought when I started doing it, 'Oh my god, I'm going to be so skinny'. But I ended up bulking up a little bit.
"I was basically on the Hugh Jackman Wolverine diet and doing pull-ups. As soon as I stopped, I think I lost like 5 pounds in 4 days. I think when I see the movie I'm going to look like a freaking marine or something."
X-Men: First Class will hit UK cinemas on June 2 and in the US on June 3.