Just bouncing a few thoughts around.......

If you assume that the person grows in proportion to the growth of the cup. The ratio of the size of the man to cup will average out to be a constant figure over a long period of time. In effect his thirst for the water will be equal to that which is held in the cup. Since he & the cup are still in the same ratio regardless of size, the net gain to drinking the water from this big cup will be exactly the same as a person using the normal size cup. Of course he still has to empty the cup or he will die of thirst & the water will go stale. Further if you liken the amount of water In both the cups to the size of the mans ego......it doesn't look so good.