Smartphone design usually boils down to compromise. The device is often designed by a committee with members having very different objectives and requirements. One compromise is the battery, which needs to be large enough to power the device components for an acceptable period of time. It often needs to fit into a space provided by the chassis or motherboard designer. It needs to be kept cool and has to have all of the relevant control circuitry included. Some batteries also form part of the NFC antenna system and others need to be integrated into wireless charging systems. The finance department have a strict cost budget but marketing want the battery to give as much life as possible so that the device looks good when they are designing posters and advertising for it.
To put this into the perspective of modern smartphones, we have seen an improvement in relative and absolute battery life over the last six years or so. Early Android devices carried small batteries that offered relatively poor battery life thanks to relatively inefficient internal components and less than optimized software. We've seen our smartphone devices acquire more and more power using technologies over the years such as higher performance 3G networking, 4G LTE networks, NFC. The number of pixels that a modern flagship device has to shunt around the screen has increased by almost a factor of ten and screens are now considerably larger, too. System-on-Chips, SoCs, have moved from ~528 MHz single core processors to 2.0+ GHz, octa core processors. Battery capacities have, for a flagship device, approximately doubled with perhaps a little more. However, battery uptime has slowly improved over the years, helped by components becoming more and more efficient.
We have also seen improvements in battery technology, whereby various businesses have been able to increase the energy density of a battery (thus for a given size, increase the energy capacity) as well as enhancements to the charging and recharging, both by incorporating fast charging technology and chemical changes designed to extend the effective lifespan of the battery as measured by charging cycles. Today, Samsung have announced a new process that could see significantly improved energy density of up to an 80% improvement. Samsung has figured out how to coat batteries with graphene, which is grown from a silicon carbide substance and combined with the lithium battery. Unfortunately, as with all technical developments with batteries, it will take some years before the new technology is ready for commercial use and of course, the battery technology can be applied to all manner of objectives and devices such as smartphones, tablets, laptop computers, through to hybrid or electric vehicles. However, all things being equal (and of course they won't be), it means we could be seeing a device the size of the Samsung Galaxy S6 with a battery of around the 4,500 mAh capacity.