It’s not just an increase in battery size which Apple reckons will see the 7 Plus last longer - it’s also the new A10 Fusion chip, which has two low-power and two high-power cores.
Whether that has a marked effect on the battery isn’t that clear, but we can report that the battery life on the iPhone 7 Plus is very good.
There is a case to be made for saving yourself some money without losing loads of features and picking up the iPhone 6S Plus instead, but the upgrades on the iPhone 7 Plus - especially the dual camera - make it a much more impressive buy than the smaller iPhone 7, and when the upgrades come to the camera software a real worthwhile upgrade.