When a Pokémon evolves, its base stats change so the displayed HP and CP increase. However, its Pokémon level and IVs do not change, so when a naturally powerful basic Pokémon is evolved, its evolution will also be naturally powerful.

Does evolving Pokémon increase stats Pokémon go?

Evolving a Pokémon will give it a serious boost to its stats and CP rating, but how high an evolved Pokémon can go is based on its CP level and stats, as they're generated when you catch it — a Pokémon that starts low will always be relatively low, even if you evolve it, and a Pokémon that starts high can come out of ...

Is there a benefit to not evolving Pokémon?

One benefit of delaying evolution is that unevolved Pokémon often learn powerful attacks earlier than their subsequent evolutions. Some evolved Pokémon also lose the ability to learn certain attacks. See our Pokédex for information on the Levels at which Pokémon learn their attacks.