In the video the iPhone 6s is dismantled and viewers can see that the battery is marked "1715 mAh". This confirms previous rumors, which have said that the iPhone 6s' battery capacity would be down to 1715 mAh from the iPhone 6's 1810 mAh.
Rumors also indicated that the iPhone 6s Plus would see its capacity drop down to 2750 mAh from the iPhone 6 Plus' 2910 mAh. While the iPhone 6s Plus' true battery capacity is currently unconfirmed, confirmation of a lower capacity will likely come in the next few weeks as the device is subject to teardowns and technical breakdowns.
It's likely the smaller battery size is due to multiple factors. Despite the smaller battery capacity, Apple’s stated battery life for the iPhone 6s remains the same as for the iPhone 6, indicating battery efficiency has been improved in the new model. Additionally, the new Taptic Engine takes up a significant amount of space that had previously been devoted to battery, meaning the Cupertino company had to cut down the battery size to compensate. Thanks Brandon!