Apple seems to somehow put a smaller capacity battery in the 6S yet maintain the same battery life as the 6, despite adding more power hungry features, like always listening Siri on battery. More efficient processors, omitting certain hardware on the smaller phone like a Taptic Engine, and 3D Touch, and maybe even removing the 3.5mm Jack to free up more space, as well as software optimized for better power utilization/conservation, and ultimately, the biggest power draw of any iOS device -- the display, which is going to be much smaller and require far less power than the 6S. And all that's before any improvements in battery tech. So yeah, it's not an impossible request.