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.