If a 40 Gallon water heater springs a leak, how many gallons of water do you have on your floor?
Answer, it could be thousands of gallons of water, the size of the tank does not matter, if the tank springs a leak, it's just like a broken pipe the water will run until you turn it off. if you are leaking 5 gallons a minute and the tank was allowed to lead for 1 hour you would have 300 gallons of water on your floor, if you went away for a week you would have 50,400 gallons of water on your floor. Have your water heater inspected to be sure it will not burst when you least expect it. The little pans that you may have under your water heater, required by code in many areas, only has a small drain line, that will work on a small leak but will be useless if you have a real burst.