I read this article about the female soldier, but
@SaraR brought me to see a point, that I didn't really think before:
The measured walking distance isn't identic for a person of 185cm and a person of only 165 cm.
The smaller person has to walk a longer distance from an individual point of view!
20km aren't 20km.
And that explaines the problems my brother got several times in the evening.
He is round about 185cm tall, she 165cm, and in the beginning of the day everything is fine, because his rucksack is much heavier than her rucksack.
But a few times they had to walk longer than expected to reach a place to put up the camp, they couldn't stay over night in the rocks around. And than in the late evening she reached her borders, the battery was empty, she got problems to follow him, even if he carried her rucksack on top of his own.
I think, the main reason was the individual distance, the amount of steps!
If he walks objectively 30 km, she walks next to him 20% more. She didn't walk only 30 km, she walked subjectivly round about 36 km. And probably that's the point!
Everybody knows, that he gets problems at the last kilometers, if he walked all the day.
I don't know the reason, but that's for sure.
And that might be the reason, why 1/5 of the body weight is recommended to boy scouts as well as to girl scouts, who usually do not hike in mixed groups.
But hiking couples usually get the experience, that it works only well, if he takes a much heavier part! And I think the reason is, that we have to count the different body size to calculate the weight, but we have to lighten the load of the smaller hiking partner once more, because over the day he has to walk a lot more steps.
And than may come on top of this different muscle structure or whatever.