I'm a graduate student at MIT and I've been doing some research on beverage consumption and carbonation (CO2) density. I noticed the same thing by accident when I was on my break and I immediately went to the professor I'm doing the work with and we experimented with different sodas and can/bottle sizes. After about a month and a half's work, we've just found out what causes this change in taste (the paper is going to be published in MITs magazine "Technology Review"). Quite frankly the results are astounding, and really, quite disturbing.

The short answer is because of these partciles in the soda called "neutrinos." The consistency of syrup has nothing to do with it (as my colleague so naively hypothesized). In soda there are many molecules and elements that are unstable (hence the bubbles), and when a certain type of element because unstable, it will undergo nuclear decay. Before this wasn't an issue, but with the some new ingredient they started buying from China, the amount of decay is phenomenal. Neutrinos are released along with Beta-positive particles in this nuclear decay. Since the decay begins when the soda is put into the bottle, there is a higher concentration of neutrinos in the smaller bottles. Neutrinos are anti-matter, and if you've ever seen Star Trek, you know that whenever anti-matter comes into contact with matter, it explodes. The reason your mouth isn't torn to bits right now is that neutrinos are so tiny that they only destroy your tongue and slowly eat away at your stomach and intestines. The reason it tastes better in small bottles is because the higher neutrino concentration eats away at your taste buds faster, which makes it taste better.

We're taking this up with the FDA and congress as we speak, and my NDA just expired so I'll be spreading this around as much as I can. My advice? DON'T DRINK SODA!