It's not like the camera juts out that much or, like on some other phones, defines the whole back/angle of the device. Apple is almost always going to want to make something thinner as long as they can maintain the same amount or better battery life in the device. And with that it's not just about bigger battery, for them it's also if the internals can be more power-efficient at the same time (A8, M7, etc.).
I'm sure that will be a part of the presentation. Apple *doesn't* just stick a bigger battery in for the sake of it. They've never done that with anything. Same as why they don't seem interested in putting more RAM in there. More is not necessarily better unless all the parts are working as efficiently as possible.