the size of tails doesnt mean anything. we know for a fact that the kyuubi is the strongest as pain stated that it has to be captured last after all the beasts{which are captured in no particular order} are sealed, or its power would make the sealing stature crumble. also, it is a cannon fact that the more tails naruto grows, the stronger he becomes. plus, a 4 taled naruto was about as powerful as a fully released 8 tails so that means the kyuubi is stronger. there is nuthing clearly stated about all the other beasts excluding the kyuubi that states that more tails means more strenght{however it is strongly hinted} . with the eight tails, it cud have merely been the great control of the guy over his bijuu that gave him the ability to be so destructive, but then again, even the two tails had a lot of control but she cudnt beat hidan and kakuzu and didnt show power on the level of 8 tails.