It doesn't really make sense. 6'3, or whatever Cameron is, is certainly tall for a fullback, but it's tall for a soccer player in general. Plenty of "tall" guys have played very, very decent fullback - in fact I can think of 7 fairly world class guys over 6 foot: Maicon (6?), Riise and Filipe (6'2?), Ramos and Danilo (6?), Abidal and Kolarov (6+), come to mind. Sanneh (6'2) did all right in JK even though he was already a lock club CD - despite playing much of his club career at Mid and FB. I would suggest that you have more tall guys at CD than FB less because they aren't quick or very good FB's but more because height is more valuable in the middle of the D clearing out crosses. If you have the quickness and skills to play FB and height, it will be a natural move inside - which is where most of the best "big men" end up. Being able to play FB, CD and midfield is good for a player if it keeps him on the field. And it seems easier for a defender to move inside as they slow/age than the other way around. Hopefully Pulis keeps finding a place for him on that crazy "mid table" stoke.
Bingo. Right now, Cameron is an unexpected luxury for Stoke. Fast enough to be a fullback, good enough with his feet to be a mid, tall and strong enough for a CB. All those traits that make him versatile also can make him into a great CB. Stokes CBs are already quite good so it's nice that Cameron can still get playing time but it's likely that in many another team he would be the starting CB.