But I never said he was incredibly powerful to begin with. And that still doesn't prove that his Nobody is exact against his looks. Roxas, Xemnas, Ansem, Namine. All have looked quite different.
Ansem isn't a nobody, he's a heartless, and he and xemnas both look similar to xehanort, just with darker or lighter skin colors.
Roxas and namine are taking appearances from two people(both born at the same moment), instead of one. So, from tat you can say that they both look like sora, which is odd hair which was given to roxas, and kairi, her dimples and persona(the kinda negative, but happy person) which was also given to roxas. Same can be said about namine.