My favorite Tarzan movie stars Gordon Scott:

He's obviously a body-builder, but not like the Jusko model I posted. Yet I can see Jusko's justification for the Tarzan built the way it is, considering that Tarzan has, in my mind, a denser mass than most men by virtue of his "superhuman" qualities...the idea that NO man could do, or be, what Tarzan becomes.

Thus, the idea is that Tarzan is "more than human"...Phillip Jose Farmer pretty much indicates Tarzan and Doc Savage and Wolf Larsen and so on are, well, mutants. I'll somewhat defend Jusko's Tarzan, only on the grounds that Tarzan's kind of "supposed to be" stronger than your normal hero-guy. Musculature is an "easy" shorthand for that, I guess. And you can justify it somewhat in the genetics of the "superhero" Pulp adventurers like Tarzan, the Shadow, and Doc Savage, who are all "more than human."

Of course, a lot of this is retro-theory or whatever, but I pretty much subscribe to it.