The hole is a breather valve to vent the bottle. The reason for the vent in the cap is pressure equalization. The reason the bottle needs to vent, or equalize is because of the variation in air pressures at different altitudes. Without the vent, a bottle sealed at 6000 feet will literally crush once transported to sea level, because of the higher air pressure at sea level. The opposite is true at higher altitudes such as being transported in an aircraft.
The reason we do not have that same vent in the 8 oz bottle is there is less air in an 8 oz bottle to be affected by altitude differentiation, and secondly the sidewall of the 8 oz bottle is stronger than the larger 16 oz bottle which has more flex in its side wall.