So was I. The underlying principles are the same for these types of movies.
I'd say the figure is hyperbole. Jason Momoa wasn't a bag of bones to begin with, and from what I hear the initial brief was for him to gain 10lbs before shooting began over the six weeks leading up to the start of production.
Here's the trailer (you get a good look at him towards the end):
Chris Hemsworth had 10 months of training for Thor and he is said to have gained a comparatively modest 20lbs. I always take these figures with a pinch of salt because the actors don't attain the required amount of mass then start filming; they train as they go for the most part. They may only hit the desired look quite late in production, so there really is no way they can issue a press release saying 'X' actor gained 'X' amount of muscle for 'X' role, because it's an ongoing pursuit.
That's why my last post extends to Jason Momoa as well.
Edit: they also said Robert Downey Jr. gained 20lbs for Iron Man...