If I understand the article "mating success" means number of sex partners per unit time for them. There is the obvious spurious correlation that guys who work out to get big may be doing so because they are trying to play the field and attract more women. Smaller guys who are married or monogamous would have fewer sex partners, and may have other priorities than getting jacked. It's like saying women with dyed hair and lots of makeup have more sex partners, if that was true it would not be clear that means they are actually more attractive, just promiscuous