What affects the amount of hitpoints each eaten candy increases?
Solution 1:
It's a formula that yields diminishing returns per-candy.
Max HP = 100 + 2.1 * Candies^0.4
(Where 'Candies' = candies eaten, obvs)
Solution 2:
Taking the derivative with respect to candies...
Each candy will give you +(.84/(candies eaten^.6)) to your max health.