The power doubles after each hit. But after the 5 turns, the power will reset.
If you use Defense Curl before you use Ice Ball/Rollout, the power is doubled as long as you stay in battle.
>Ice Ball does damage over five turns, increasing in power for each consecutive hit. The power will reset after five turns or if Ice Ball is interrupted. If the Pokémon uses Defense Curl any time before Ice Ball, Ice Ball's power is doubled as long as the Pokémon remains in battle.
Hope This Helped :D