Of the 4 phones produced with the general 6 design, the 7 has the joint worst battery life, despite having the largest size battery of them, and a chip specifically designed to be more power efficient (having the two pairs of cores, one of which are specifically for low intensity use).
Battery size:
1,821mAh - iPhone 8
1,960mAh - iPhone 7
1,715mAh - iPhone 6s
1,810mAh - iPhone 6
Battery endurance (from GSMarena*):
66h - iPhone 8
62h - iPhone 6s
61h - iPhone 6 & 7
*This is assuming a baseline of 1h talk time, 1hr web browsing and 1h video playback per day, you can adjust to a more realistic use case on their website, but as this is just for relative purposes it works well.