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Well, Braeburn has back leg strength from apple bucking, but he also does farm labor so that would even out his core muscle. Soarin is an athlete, as a pony he has strong wings. Though changed to (wingless) human this can be translated to track, jet flying or car racing (to name some interpretations). So assuming it's track, Soarin has more leg strength. As a runner he would not necessarily need to be bulky as a small frame allows one to run faster. Or even with other interpretations not as much arm strength is needed, thus his arms are weaker. Now through your (wonderful) depiction they both seem to have the same muscle mass. But if one was to use the information presented, I would say Braeburn gets the pie, because of the daily farm labor. Working a farm would give him more muscle mass and strength.
Soarin is a top athlete and performer, used to bursts of maximum energy over short periods of time. Braeburn is a farm hand, used to working hard from dawn to dusk. Braeburn would definitely win a marathon or a race to haul 16 tons across the farm, but I think Soarin's got this one.
Wonderful physiques... And in a contest of pure arm power Braeburn would win, if only because his muscles are developed more for heavy labor than Soarin's more specialized and speed-based physique.
With that said, if they were full on fighting as opposed to just this friendly competition, Soarin would have the upper hand because he'd be able to deal damage too quickly for Braeburn to effectively counter against.