20oz are more expensive per volume because they sell faster. There's less of a demand for larger sizes typically go flat too fast for people unless they're having a party or something, and even in that case they don't have the convenience of being able to drink from the bottle.
Do you leave your soda uncapped or something? I buy almost exclusively 2 liter bottles and they very rarely go flat on me. I'll drink one over the course of about 4 days.