Note, I didn't say it wasn't enough only that no wonder it seemed light in weight. For all we know you could be consuming a fair number of calories. We don't know what is in your TUNA SURPRISE -- this sounds like something one could really pack some fat and protein into. (I take it this version isn't full of mayo, though.) Sounds like it works for you. My comment wasn't meant to be a judgement of your diet. Sometimes when I exert myself for days at a time especially at altitude, I have a light appetite. Maybe this is due to adrenaline. Who knows. Then when I get home, I EAT! BTW, have you tried Probars?
In the case of the 14 year old boy, hardly a shocker. Sedentary 14 year old boys often go through a seemingly enormous quantity of food. You are fueling a growing body and taking on some serious activity. Good on you for being so into this at 14! :)
In terms of metabolism, something will "give"…eventually. I would be interested in whether you have done any month long trips and whether your caloric needs have changed. On average, one needs roughly a 4000 calorie deficit to lose a 1lb of fat. It doesn't surprise me in the least that you maintain good condition for several days on your diet.
Efficiency of movement enters into the equation as well! Some people on the trail walk like elephants. Some move like gazelles.
Just a side note: An increasing body of research suggests that those who consume fewer calories, generate less free radicals and live longer. (I realize there are confounding factors -- ie. they would be less likely to be overweight!)