I would like to add geography into the discussion. While building the dam is one thing, and having benough water is another. The reality is that most basin areas that are exactly of right size are for most part already dammed. So the area is also partly a function of the size and shape of the area on the other side of the dam.

It would be economically very expensive to build the entire area surronded by a dam. The aim is to use the natural features as well as possible. This encourages the slight exageration of the area inorder to maximize power output. The area is softof "free" anyway.

I would like to add geography into the discussion. While building the dam is one thing, and having enough water is another. The reality is that most basin areas that are exactly of right size are for most part already dammed. So the area is also partly a function of the size and shape of the area on the other side of the dam.

It would be economically very expensive to build the entire area surronded by a dam. The aim is to use the natural features as well as possible. This encourages the slight exageration of the area inorder to maximize power output. The area is softof "free" anyway. Having more area afford you to have more turbines in parallel before you start to affect the water level of your dam.