Lembas

[FairLadyGoldberry]

Well, in the book it states

It [the lembas] did not satisfy desire, and at times Sam's mind filled with memories of food...

So maybe the hobbits felt the need to physically eat larger quantities of lembas, like they were used to doing with regular food when they were very hungry. Maybe they ate more then they needed to b/c just eating a small piece did not fill their strong desire for food.