Sportbilly

More detail:

What you're seeing my Soupy friend, is the effect of surface/mass ratio.

Lower water level in a bathtub doesn't make for much of a reduction in the surface area, hence, the smaller volume of water cools down faster as it contains less energy.

In theory you are right, your reservoirs would lose more heat, so long as they either contained less water, or had greater surface area, but as the Hydro-Pimp pointed out, your water won't notice much of a difference, although I would suggest that it's not because of the time spent in the reservoir, as much as it is because the plastic is a lousy conductor of heat.

After all, the reservoir ALWAYS has water in it at higher than ambient temperatures, time spent in the reservoir by individual water molecules is irrelevant.

EDIT: The radiator itself is what you're suggesting taken to it's logical limit. A radiator is simply a small volume of coolant, with a large surface area (happens to be made out of a much more heat-conductive material also) If you want improvement that's noticeable, add another radiator, but you'd best make sure it's outside the case (or at least exposed to cooler air) to get the best out of it.