uh, yeah they did. In 1945 they had nearly half. Today they have a little under a quarter.
Nope, sorry.
In 1944 (last complete year of war)
Production (GNP in dollars)
USA - 1500 billion
UK/USSR/Germany/Italy/France/Japan - 1700
Thats excluding the reast of the Empire, plus all the other world countries!
USA had about 25% of world GDP.
Also manpower issues - USA would soon run into the problems Britain had, you cant use the men in the factories if they are in the army...