There are certain things that have to be customized to the body for fit reasons. There are other things that are lever arms, and if you are already on the large side you probably don't need more leverage.

Think handle bars. Maybe they need to be wider for a guy who is 6'6" to fit or be comfortable, but does a guy that size need wider bars to muscle the bike, compared to a person 5'6"? He is already stronger.

Think golf clubs, a big guy needs them fit to him, but he already has a bigger motion, and more power, would he necessarily want longer clubs?