It is true that a ball of snow increases in mass, volume and size; when it is rolled over. But in this case, the growth in size of the ball happens because of accumulation of material from outside. In case of growth in a living being, the growth happens because of accumulation of material from inside which is brought about by cell division and formation of new protoplasm.
Thus, growth in a snowball is altogether different when compared to the growth in a living being.