> You can’t really exercise enough to offset the food you can eat in a day

This is not true. There are plenty of activities that a healthy, enthusiastic person can do for quite a few hours per day that burn calories rapidly, and it takes serious effort to eat enough to offset the calorie usage. Think distance running, skiing, cycling, rowing, etc.

Sometimes people go to extreme technological measures to optimize their ability to consume calories (see recent record-setting marathon results). Sometimes people rely on extremely calorie-dense foods (butter, nuts, etc) so they can carry meals with them that are not wildly calorie-deficient for their long, unsupported travels.

When people are burning calories at this rate, they are probably not doing it for exercise — they’re probably either doing it because it’s fun or it’s part of an adventure.

I’m suspect that, if someone is overweight but in good enough shape to cross-country ski for six hours a day, then even if their body tends toward telling them to eat too much, they could probably lose weight by doing so for many days in a row.