Coming at this from a math background, I look at this as a two-variable optimisation problem. For a fast marathon you need both high aerobic capacity and sufficient strength/endurance to use that aerobic capacity for the whole marathon. Sketching out the argument (without deriving the physics by formula):

Aerobic capacity: I assume the fastest possible time you need to run at your aerobic capacity the whole way as running at anaerobic levels for substantial length of time would lead to cramp/breakdown/not finishing. That is your aerobic capacity is a limit to your power production, and to get the maximum speed from fixed power would lead to wanting to minimise your weight.

Strength: The runner would need to have sufficient strength to be able to sustain their maximum aerobic speed over the whole of the race. They may also need a little more in case of wanting to be able to run faster patches for tactical reasons, managing their speed downhill, for injury prevention (especially through training) etc. There be effects of a larger muscle being able to store more glycogen which would improve endurance.

Proposed solution: The runner needs to be as light as possible subject to having sufficient strength to maintain maximum aerobic speed and carry enough glycogen. While that doesn't lead to an exact answer (and never will) the line of reasoning could be used to determine what is limiting a particular runner.

Intuitively this make sense to me as the shorter the race the more anaerobic/strength based and therefore the larger the athlete, which would appear to be the general trend.

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