In response to a discussion on another blog, I sat down to do some thinking on weight and weight loss. For those who cite mass-and-energy balance principles, "calories in = calories out" where "calories in = food intake" and "calories out = exercise" is so oversimplified that it's actually wrong.
"Calories out" has to be adjusted for base metabolism rate. This is affected by growth rate, for the youthful, and such things as pregnancy, lactation, and menstruation in women. The efficiency of exercise also depends on muscle proportions: those with comparatively more/larger muscles (such as men) will be more efficient at expending calories with exercise. Both of these are also affected by body shape and proportions. People generate heat in proportion to their volume or weight, but get rid of it roughly proportion to their surface area (affected by shape). Volume goes up much faster than surface area.
This also helps account for why obese people find it harder to exercise. The body only tolerates a narrow range of internal temperature. A person who can't shed heat as fast through sweating and rapid breathing as he (or she) is generating it through exercise will be psychologically compelled to slow down or shut down the exercise. It seems to be one of those limits on what your body will let you do with it. Those who have found a reasonably effective balance between heat generated and head expended in extended aerobic exercise may find incomprehensible that an overweight person can't, or won't do the the same exercise and get the same benefit. But it's true. Different people sometimes really do require different approaches.
"Calories in" has to be adjusted for digestive efficiency (the ration of calories absorbed to calories ingested). This tends to change with age. I've seen studies that show it's affected by the types and proportions of gut bacteria. It's also affected by the kinds of food one eats, since some foods are more easily digested than others.
Altogether, these suggest that the efficiency of increased exercise in losing weight tends to do down as one gains in weight. Adjusting the diet would seem to be the more effective approach.