Also, if it is a really old water heater, the tank may be filled with sediment and causing the smaller amount of water to boil over, as the elements will heat up smaller amounts faster.
What Is That Hissing Sound From The Pressure Relief Value On Top Of Electric Water Heater?
If it's coming from the relief valve then you need to replace the relief valve to stop the water from running out of it. You would be wasting water and power from the heater.