Welcome George [smiley=laugh.gif],
Ive noticed that myself too since Ive been imbibing from our Plunger. Dont know why this is... Could be that with a larger volume of hot water the cooling temperature gradient is not as steep and therefore extracts more from the coffee in the same amount of time due to the higher overall temperature average :-?. Maybe the trick to beat this effect is to allow for shorter brew times as the quantity goes up.
One thing you have to be careful of, and I might be teaching you how to suck eggs here, is that you must NOT pour freshly boiled water over your prepared plunger/coffee.... Itll burn the coffee and give it a much more bitter flavour. Ive found that if I turn the kettle off when it boils, head over to my coffee bench, measure and grind the beans then add to the Plunger, the water has cooled down enough (to around 93-94 C) so as not to burn the coffee.... This then gets added and a minute or two later I have a brew that is equivalent to the nectar of the Gods :). No bitterness, just sweet, flavourful coffee..... Perfect 8-).