Surviving for years on a single food source like the tailies that is so carefully rationed would not lead to strong or plump people as seen in the tail. If anything, their constant malnutrition would make them easy to beat in a fight.
It is never explained how the tailies get water. They are assumed to live without water. This would be impossible, since a human can only survive without water for about three days. Drinking urine in the absence of water would provide an extra day or two of life, but any more than that would cause kidney failure and death.