One reason babies sleep so much is that human growth factor, made by the pituitary gland, is secreted during sleep in higher concentrations than when the baby is awake.
How the body and brain coordinate the process is not fully known, but for the first few months of life, a baby sleeps most of the day, with short periods of waking, and grows quickly.
As the sleeping time gradually decreases, growth slows.
A baby who weighed seven pounds at birth would typically weigh fourteen pounds at five months, twenty-one pounds at eleven months and twenty-eight pounds at twenty-four months.
So you can see that the weight gain curve flattens out tremendously.