Sure, Doga's definitely taller. But one of the most characteristic aspects of Doga is his body, one that has been sculpted with long hours in the gym after being raised like a mangy dog. He trained in weightlifting, martial arts, boxing and marksmanship, and is clearly a perfectionist.
Any wonder why Ghajini-boy would be perfect for the part? The dog-mask could even hold in his drool. And Aamir-Doga action figures would fly off the shelves.