Yes. But after 2.5 million years we are no longer the same....
And if we have a living relative somewhere on Earth? It stands to reason after 2.5 million years? They are going to be different as well.
Both from us and the common ancestor. But we will share many traits as well.
Its just not realistic to say that Paranthropus is too small in Africa 2.5 million years ago to be X,Y or Z now. Look at Gigantopethicus, look at the Hobbit. Wide swings in sizes among primates.