Maybe you want to have a cup of tea instead? Way more cheap and healthy. Or buy some off-brand soda. It is just as much garbage as coca cola but at least it's cheap.
Diet soda contains sweeteners that are suspected to cause cancer. Granted, you'd have to drink a couple of liters of diet soda a day before you need to be seriously concerned, but tea has one big advantage: It contains as much sugar or sweeteners as you add to it, so there's that.
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.
if you're buying coke in america, you should get the 12 packs at grocery stores instead. it's anywhere between $5.99 to $8.99, which is less than a dollar per can
In my highschool psych class we actually went to an elementary school and did this experiment with the kiddos. It was a while ago but if I recall correctly, 9/10 times they thought tall = bigger. I bet some people never grow out of that mindset or at least at first glance our less smart brain goes “tall is big!”