It's rather close to 1/6 of a US gallon, so if sold in a 6pk the pack would be 1 gallon. No idea if that's the real reason but it makes sense to me.

I think for my money the X series of games (X3: Albion Prelude and X4: Foundations) are some of the best singleplayer space empire-building games out there. They've definitely got their faults, but they're highly moddable and people make some overhaul mods that let you have more or less the exact sort of experience you want to have in those games.