To be fair, I haven't read the paper, but it seems like body size and skills/knowledge are not the only hypotheses that predict hunting competence. Another might just be motivation, mainly mediated by number of dependents. In addition, Kaplan et al. have found that hunting return rates tend to peak at 4o years old or so, long after human males have reached their peak body size, so I'm unclear as to why they would even consider something like body size.
How long does it take to become a proficient hunter? Implications for the evolution of extended development and long life span
Michael Gurven, Hillard Kaplan and Maguin Gutierrez
Journal of Human Evolution Volume 51, Issue 5 , November 2006, Pages 454-470
Abstract: Human hunting is arguably one of the most difficult activities common to foraging peoples now and in the past. Children and teenagers have usually been described as incompetent hunters in ethnographies of hunter-gatherers. This paper explores the extent to which adult-level competence is limited more by the constraints of physical capital, or body size, and brain-based capital, or skills and learning. The grandmother hypothesis requires that production is an increasing function of size alone, while the embodied capital model stipulates that production is a function of both size and delayed learning. Tests based on observational, interview, and experimental data collected among Tsimane Amerindians of the Bolivian Amazon suggest that size alone cannot explain the long delay until peak hunting productivity. Indirect encounters (e.g., smells, sounds, tracks, and scat) and shooting of stationary targets are two components of hunting ability limited primarily by physical size alone, but the more difficult components of hunting—direct encounters with important prey items and successful capture—require substantial skill. Those skills can take an additional ten to twenty years to develop after achieving adult body size.