Is a tall, dominant man usually the top?

Please shed some light on the gay lifestyle. As a straight man myself, I am very interested and curious about the roles gay men take. As I observed a gay couple in the book store, I noticed that the tall guy was quite dominant and commanding towards the shorter, younger guy. I did not dare to ask them, but I thought that the tall dominant must be the top, and the submissive man the bottom. The 'bottom' was making a call on the 'top's request for example. Then the small man asked if they should visit another book store but the tall man said no. On the other hand, the larger man had long feminine hair. Am I right in assuming that the tall, dominant man is the top?