Second, the Shazam we see in the movie is not only huge, but his muscles are clearly defined while in his suit. He looks like an action figure and while Zachary Levi may have had good muscle definition with his shirt off, putting on a costume, even a tight one, is going soften that definition to a degree. So if the character’s muscles were going to pop in the suit as they do on the pages of DC comics, the padding was necessary.
Article source: https://www.cinemablend.com/news/2485805/zachary-levi-now-admits-his-shazam-suit-was-a-little-padded