Hunky werewolf Jacob Black, played by Taylor Lautner obviously had to get pretty beefy for New Moon. The rest of the cast has never had to be very physically fit until the third installation. Maybe because they have to fight a army of vampires?
Twilight's Elizabeth Reaser told MTV news they had a hard time keeping up with Taylor. She said he, "doesn't even have to train. He could just stand here and do a back flip for no reason with using about 5% of his energy. He's so fit. "
Then she had some stuff to say about skinny Robert, "This stuff is really hard to do and we trained for hours and hour and hours and he worked really hard. And he's a great actor, so he really committed to what he was doing."
Robert is so tiny compared to Taylor. Taylor did have a head start on the rest of the crew because he had been training for a while to get his body the way it looks now before they had to even start.
But, check out the video here.