Stats

Armor, magic resist, mana and attack speed are the same at every star level.

Nunu & Willump

1 star: Bite the current target, dealing 450 magic damage. If the target has less Health than Willump before the bite, deal additional 50% damage and becomes true damage.
2 star: Bite the current target, dealing 700 magic damage. If the target has less Health than Willump before the bite, deal additional 50% damage and becomes true damage.
3 star: Bite the current target, dealing 1600 magic damage. If the target has less Health than Willump before the bite, deal additional 50% damage and becomes true damage.

5 patches changed Nunu & Willump, read out of Riot’s game files.