When the guy with the forklift runs the forks into the pallet load he'll only punch one or two quarts. If he hits gallons he gets at least one, maybe two. There is a big difference in the fire hazard between a quart of solvent on the floor and a gallon.
Same thing when the truck turns over on the cloverleaf. If ten containers pop open, would you rather they be quarts? or gallons?
I'm waiting for a fuel supplier to come up with a 3 qt. bottle although I suspect the hazmat folks have set the size a liter or more for the special handling.