That'd be if whites stuck to the one-drop rule. But if it were reversed, and having one drop of 'white' blood were enough to make a person white, then if all white people in a single generation mated with someone from a different 'race', they in fact would be more than doubling up their numbers in one generation (considering a birth rate higher than the equilibrium number).
It's all in the sociological definition.
Now, if races were defined based on something more precise (certain biochemical markers), then the possible distributions are almost infinite. Biochemical markers are, as a rule, neither entirely recessive nor entirely dominant.