In the episode Sky High Gym Battle of the anime, when Ash is in Hoenn, he faces Winona. Her Swellow is a lighter shade than Ash's. It is also bigger. I know that her Swellow is shiny, as its coloring matches Swellow's shiny sprite, but is there any explanation for it being so dang huge? It's at least two-thirds as big as Ash's.
Here's a pic of her Swellow. It doesn't have Ash's Swellow in it, but some quick Googling should work fine.