Another way to make it more fair. Give Japan all of the Western hemisphere US territories.
The way you have it set up is:
Ger + UK + 50 % of US.
vs the original setup of GER + Jpn.
In my estimation, Ger + (any other country) makes it an interesting and usually fair game. Adding any more to that will unbalance the economics and/or require a military offset. You may want to also shift the order of nations in the round to prevent the taking of capitals at the game outset.