Jason Snell offers an interesting analysis on the daily battery expectancy for iOS devices.
Over the years I’ve said numerous times that when it comes to battery life on iOS devices, Apple appears to have a target battery life in mind and builds its hardware—a balance of power-saving software, hardware efficiency, and battery capacity—to hit that number.
In other words, Apple thinks we need, say, a minimum of 10 hours of daily battery life and as they design each device, they make sure the battery is big enough to support that goal, but no bigger than that.
The one outlier to this model is the iPhone 6 Plus. Why? Because they weren’t driven by the need to make the phone as small as humanly possible. They had the luxury of design space, of battery headroom.