whats the point of having a big resovoir?

Solution
Larger reservoir means a larger volume of liquid. Take that much longer to heat all that water, so I suspect a better cooling solution.

-Wolf sends
From what I understand, having a reservoir is better than not since it allows a place for trapped air to escape to, and lets you see when the water-level is low.

The size of the reservoir itself doesn't matter all that much, other than aesthetics, which is purely a personal thing. Size does not affect heat dissipation in any appreciable way.

(Most of the above was paraphrasing Jayz2cents)