What the designers did was break up the big Japanese sea zones into a lot of little ones but they left the sea zones close to the US wide open which encourages a policy of harrassment by a smaller US navy. But, the US is unlikely to make it very deep into Japanese controlled sea zones without spending every dime on the Pacific.
However, with a small naval investment the US can usually claim the Midway, Wake, Hawaii, Solomans National Objective and keep a decent portion of the Japanese navy tied up.