“It was a standard bodybuilding program, so we would do, for example, back and biceps, chest and triceps. We would focus solely on getting a little bit of strength, a little bit of size into his body,” Sana Shirvani, Jonah’s trainer, explained. “We got him to that point, and then obviously, if he continued training in that capacity, he was only going to get bigger and bigger. I remember the director coming up to me and saying, ‘We are not after the Incredible Hulk, we want a strong sailor boy.'”

Source: https://www.buzzfeed.com/laurengarafano/the-little-mermaid-behind-the-scenes-facts