Yeah that was a running joke. Sorbo and the producers agreed that, during the twentieth century, because of movies, Hercules had become unhealthily associated with steroids and they wanted to move the character away from that and into a fit look but one that didn't scream steroids. Of course, the result was the running joke, "You're Hercules? I thought you were bigger."
And, yes, Hoechlin could benefit from the same sort of running joke.