The power of Eruption and Water Spout is determined by taking how full the HP bar is, as a percentage, then multiplying by 1.5. So whether you start at 320 out of 320, or 380 out of 380, that makes no difference at first: full health always means 150 power. But then if you take 160 damage, the 320-HP bar is now halfway gone, so its Eruption drops to 75 power, while the 380-HP bar has more health left, so it's still in the green and an Eruption there would be for 86 power.

Obviously, if the Special Attack stat is higher, that does affect how much damage you can deal with a move of a given power.