Apple's iPhone 7 and iPhone 7 Plus are said to sport higher-capacity internal Lithium-Ion batteries than their predecessors.
No official battery capacity specs are listed on Apple's website, but they do say that the iPhone 7 and iPhone 7 Plus are supposed to get 2 and 1 additional hour(s) of battery life respectively, in what are supposed to be after effects of the new power-sipping A10 Fusion chip combined with the efficient iOS 10 firmware.
On the other hand, a higher-capacity internal battery may also have something to do with it.