This phone is a battery beast... 5+ hours of SOT, with a screen this big from a device this overall size and thickness.
To be realistic though, the OP is comparing an old device with a brand new one. ALL batteries lose capacity over time with repeated charge-discharge cycles. This is why I will always prefer a replace-able battery if given the choice. Buying a new device every time its internal battery loses capacity is not an option for me. A more realistic comparison would be to compare a relatively new S7, or one that recently had its internal battery replaced.