Bigger screens = More space for Bigger Batteries
The iRumours are that the next iPhone will have a bigger screen. Now, I might be wrong but the biggest benefit I can see with a bigger screen is that there's more room to fit bigger batteries which means the batteries will last longer. The unspoken shame with the iPhone is that the batteries only last if you don't use it. Apple, obviously doesn't make a big deal about battery life, now, when it sucks, but I bet they do at their next launch.