In reality, May's experience plus Glaceon would whoop Dawn and Piplup. But since Pokemon isn't reality and the fact that it's up to the writers to decide on these sort of things, Piplup would win ragardless of what Pokemon May used (within a sensible limit, of course). Dawn had to win a contest at some point. That would be the only explanation why Dawn won in the first place.
But back on topic, there's more chance of Piplup winning if May uses Blaziken. While Blaziken has raw power and experience, Piplup has type advantage for both of Blaziken's types. Not to mention that Piplup's size in comparison to Blaziken would make it hard for it to land a hit, and as we all know, missed attacks means losing points.
Really, it's the battle format that brings out these sort of outcomes, not the Pokemon in battle.