My experience is that capturing and holding smaller continents early on is best. Capturing and keeping North America, Europe or Asia early on tends not to work well when I play.
In South America, you can get lucky and take North America in 1 or 2 turns. Since North America shares a front with South America, you end up with the same number of fronts you would have if you possessed North America alone, and in a very defensible position. The fewer fronts you have, the more interior you have that doesn't need to be manned with troops.