I've had every iPhone and don't remember how my battery was on each 1, but I remember with the 3GS I would only go down 10% after an hour.
My iPhone 4S battery will go down 25% after an hour.
I feel like a bigger iPhone means bigger battery, but if they keep adding more features and better processors and screens, how will the battery life get better?
It seems like battery life/technology will never be able to properly handle a phone's features.
iPhone 3GS was a clear improvement over iPhone 3G. iPhone 4 was definitely a clear improvement over the iPhone 3GS. I don't own an iPhone 4S, so I don't know what that's like.
And besides, adding better processors and screens doesn't mean battery life can't get better. Every few years everybody changes from architecture, which means that chips will deliver a much better performance, at lower clock speeds and thus use less energy. Also, every once in a while chips become smaller (like switching from 45 nm to 32/28 nm lines) - performance stays the same, the chip becomes drastically more energy-efficient.
Same for displays: remember that IGZO display? It's basically just a LCD display with some small adjustments: these adjustments, however, allow for a higher brightness compared to 'standard' LCD displays - which means better battery life (because less LEDS are needed for the same brightness).
And oh, battery life can improve - but Apple is just obsessed with thickness. The "iPhone 5" is rumoured to be only 7.4~ mm thick, compared to the iPhone 4S' 9.3 mm. We're talking here about almost 2 mm.
What does that mean? Well, that means Apple has managed to give a 7.4 mm "iPhone 5" (with new features), the same battery life as the current 9.3 mm iPhone 4S.
Theoretically, this means that they could deliver an "iPhone 5" with a thickness of 9.3 mm that could last for a couple of days (compared to currently one day (depending on your usage)). They could also have settled with (for example) 8.3 mm: that's still a millimetre thinner than the current iPhone 4S (and I believe it than would be the thinnest smartphone on the market (not sure though)), and they could still offer vastly superior battery life compared to the iPhone 4S.
But they won't do this - because they want to make their devices as thin as possible.