Look at the size of Ash´s Snorlax in the early epidodes, it was HUGANTIC, and recently in the battle against greta, it was just a little bit bigger than Medichem. I remember that in the Clair epidose, when Snorlax beats Kingdra, Ash goes to hug him and the difference between sizes is enormous, and look at its Size in the Battle Arena, we just have a Pigmy Snorlax. The point is that the writers tend to forget the proportions of the sizes. I don`t think its a different Articuno, cause I`ve never seen two Articunos at the same time in the Anime, It could perfectly be the same Articuno from Johto and from Shamouti. And Articuno in the Power of One was just shooting Ice Beam over Ice Beam over Ice Beam, I would not call that "More Powerful". And if Charizard could beat it it was because its hard trainer, and another important factor, its type adventage.
There´s no anime inconsistency in the fact that there are only one of the trio of birds, cause as I said before I´ve never seen more than one at the same time. You could say that about Lugia, and also about Lati@s, but not with the birds. Lawrence clearly said that