With increasing power of processors and space, people are tend to install more applications. Powerful cameras, incredible audio quality, videos over 4G are all contributing towards more usage of phone. So phones need more powerful batteries
Many well known brands are introducing 4000 mAh to 5000 mAh battery in phones. Many less known brands are also providing 10000mAh batteries.
On the other hand latest release of iPhone 7 plus is offering 2900 mAh only. It is sure that they are increasing battery capacity but not huge like other players. iPhone is more of a status symbol and has a unique clientele. it is unlikely that they will introduce 4000 or 5000 mAh battery in near future.