Big versus Little Dams
Since the pressure is what exerts the force on the dam, and since pressure depends only on depth (height) of the fluid and the type of fluid (DP = rgh), if both dams have the same water depth behind them, then both dams have to be just as strong.
However, the energy stored behind the dam depends on the amount of water. Clearly the dam with the most water contains the most energy (and the most danger).