Sounds like a good point, however, what was the price differential between the two? If the double-capacity version cost twice as much, then obviously not many people are going to buy it.

I think it'd be interesting to have an experiment where some flagship phone (like the Galaxy S*) is sold in two versions, one with double the battery life but a little thicker (and cases for both sizes are available), at the exact same price, to see which one people really prefer. Personally, I'd certainly go for the double-capacity version, but considering how popular iPhones are, I'm under no illusion that my tastes represent the mainstream.

When I last brought a Motorolla phone, the Maxx was a bit more than double the price of the similar, but thinner one I brought.

Yeah, that would explain why the thinner one sold better. It's not because people value thin, it's because people value saving money.