About ten years ago, I was thinking about the theory of natural selection and muscle mass. I wondered from an evolutionary standpoint, what was the point of big muscles on a person? Muscle mass is "expensive" in the sense that it takes calories to maintain it. In addition, muscles that are too bulky might decrease movement and speed. It would seem that that being strong and having smaller, more compact muscles would be preferable to having the same level of strength yet having large muscles.
My general idea at the time was that big muscles served as some sort of visual indicator to others, and that this had implications for reproductive success. Later, after reading Geoffrey Miller's book, " The Mating Mind," I found a theory to match my idea. Miller discussed the theory of sexual selection, and how things that may seem expensive and wasteful from the physical side of natural selection are often important in sexual selection and mating.
Some background on Miller's ideas about sexual selection can be found here. Miller talks about indicators being subject to the "handicap principle," that is they must have high costs to be reliable. If my theory about muscle mass being a record of one's "hunting success" is true, then muscle mass would also satisfy the handicap principle. It takes months of physical effort in acquiring game (resistance training) and ingesting some of the game (protein and fat) to build muscle. In Paleolithic times, this could not have been "faked."
Hence, the fact that muscles are showy and expensive may tell us what their ultimate function is: an ornament in the game of sexual selection.