"Comer até ficar triste" could be two things. The first, would be to eat very much in a sitting, to the point of getting sleepy after the meal.
That is said also after somebody having eaten heavy food too much, for example, after a "feijoada" which gets the eater full and sleepy as well.
Would be the equivalent to the lions, which after having feasted up over a prey they get so sleepy that they lose the rest to other animals.
''Comer até ficar triste" would involve the slowing of metabolism, sort of. Not a good thing, for example, when someone hits the road !