assuming the amount water needed is directly proportional to the size of the muffin. the amount of water needed for the smaller muffin can be solve by first solving its size,
size of small muffin = 2 / 5
amount of water for the small muffin = ( 2 / 5 ) ( 3 / 5)
amount of water for the small muffin = 6 / 25 cup of water