Possible bad cooler, or bad testing?

The coolers here suck, but I finally found one large enough for all-grain (~6G). Due to a lack of DME, I tried making an all-grain starter. I did this by basically scaling a normal all-grain, hop-less beer to the size of my starter (around 1 QT).

What I noticed, and is probably obvious to many here, is that the small quantity of grain and water did NOT keep temperature for the hour, and not near enough to salvage the starter. As an experiment I tested 1 Gal of water (no grain) at strike temperature, and witnessed a 18F drop. The cooler is much warmer on the outside than previous ones I have used, and seems kind of crappy.

Ok, so I have decided on another way for the starter. My question now is this: does my cooler suck, or am I using too small quantities. For example, 1G of water drops quickly, but maybe several gallons plus grain will drop more slowly. Do my observations reflect what is expected, or do they reflect a poor cooler?

Yeah, the fuller the cooler, the less heat it will lose. Air will transfer more heat than water (I think that's the explanation).