The psychology of milk

My bride and I have noticed a quirk of psychology regarding the consumption of milk in our abode. Based on the recommendation of friends a couple of years ago, we prefer Horizon Organic milk, and originally bought it in half-gallon cartons, since this seemed to be the largest size the stores that carried the brand would stock.
At some point, gallon sizes appeared, and we began purchasing the larger size. This is when the quirk began:
We drank less milk when we had the gallon container than when we had two half-gallon containers.
I’m still trying to figure out why this is so. As a result of our not drinking as much milk, we found ourselves reaching and going beyond the expiration dates on the gallon-sized milk, something we’d never done with the half-gallon containers. (It should be noted that milk in cartons often have farther-out expiration dates than milk in semi-transparent jugs.)
Switching back to half-gallon containers, we’re back to drinking more milk than when we purchased the gallon sizes. I’m somewhat baffled.
Amateur shrinks, have at it.