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It's like a can of worms!
He is indeed much fitter ( / faster) than me, usually around 15-18kg heavier than me yet can run a good 4 minutes quicker in a 10K race. But what about the male vs female difference? EG, a 25 year old female running a 10K in 50 minutes scores an age grading of 60.7%. A male of the same age finishing in the same time scores 53.7%.
The heavy bloke carrying nothing vs the skinny bloke with the weights. The skinny bloke would be fitter? He's carrying 40kg more than he's used to, or like you say, 66% of his weight. The 100kg bloke would be used to the extra KGs. What if the 100kg bloke lost 40kg, 60kg man remains the same, who would win? If neither of them loses or gains fitness in the meantime the 100kg man would win. I think.
Going back to the original question, what about futher extremes? A marathon runner finishing in 2:30:00 has got to be considerably fitter than one who does 4:30:00?
Men v Women is even trickier the World record holders (according to wikipedia) are 56kg and 54kg respectively. Who is fitter?
As I said earlier you have to define what fitness is before you start. Otherwise you are just asking who is faster? We have no means of knowing whether Paula Radcliffe could run for 4hours at the same speed as Mrs. Average Runner and then how long each could keep going. That would be a measure of their endurance fitnes. We already know Paula has better speed fitness.
So what about strength fitness. How many KGs could Paula Ratcliffe do in a squat compared to Mrs Average Runner. How many squats can they do?
And Flexibilty? Maybe Mrs. Average Runner teaches Yoga as her day job.
Conventional wisdom says that if you can run a marathon in 2hours you can run one in 4hours. I've walked 50miles in 17hours. My marathon time is quite a bit quicker than 8.5hours.