They selected 18650, because it was widely used and available in large quantities. They designed 2170, so I guess it is best size. Manufacturing cost are lower for 2170 back, because it needs less cells. I figured out one volume advantage for larger cell: Center of cell might not be as tightly packed as rest of it. See:
(He should have used gloves. Comments say he made other mistakes, but this shows what is inside.)
Capacity / volume is roughly same for 18650 & 2170. Change in chemistry will increase capacity, but they don't change it every year.