First off, that's a great modeling tool you've got there. It must have taken some time to put it together. Good job and many thanks for sharing with us.
I would have thought Roman might been thinking of a smaller person than ~163lb or female - lower food requirements due to body weight (and basal metabolic level in the case of many females).
Though at some point the trade-off b/t physical strenghth to carry a heavy pack and food reaches a limit and it goes the other way (i.e., counterproductive). I guess your model predicts this point to be ~163lb.
Second, we should ask Roman Dial "Why did you say large feet?"
When i read his words, i very tentatively thought (since his words provided no clue to what he was thinking when he "penned" those words):
a) balance (helps with a heavy pack, though trekking poles work even better by providing a wider base with more "legs"/supports - nearly always two or three in contact with the ground at any point in time).
b) larger surface/contact area needed for a heavy pack (65-70lb) in order to reduce pressure on very soft and muddy trail surfaces to avoid sinking in as deeply.
c) perhaps he noticed tussock spacing in the Artic and was thinking of a larger foot spanning the gaps b/t tussocks, thus making walking easier???
d) was it solely the sum cumulative effect of the linear distance covered by each stride and the energy expended to take a step (not merely leg length, but the distance from heel plant to ball-of-the-foot lift-off with - a couple of inches with every step could really add up over ~2160miles or so)? [i would guess that the problem is more than just the energy expended lifting the weight of a larger foot and larger shoe/boot - again, even simplifying and considering foot size/weight alone, apart from other related criteria, it's a min/max problem.]
I really have no idea if he meant any of these possible reasons, but there has to be [a] reason[s] and he knows what it is.
Does your model take into account these phenonmenon? I'd be surprised if it did (other than perhaps one [or both] aspect[s] of 'D' - foot/shoe length or weight, maybe you've considered both - i think both need to be considered).
So, Mr. Dial, would you care to educate us, please?