iPhone 13 Pro battery life performance and the rumored upgrades coming to the iPhone 14 series.
Starting with precedence, the iPhone 12 actually had a lower-capacity battery inside it than the previous-generation iPhone 11 (2,815 mAh vs. 3,110 mAh) and yet Apple claims both offer up to 17 hours of video playback despite the capacity differences.
The iPhone 12 Pro Max also had a smaller battery capacity than the iPhone 11 Pro Max (see table below).
Apple can make this battery life claim because of the improvements made over each generation, and it is likely the same can be said for the iPhone 14 series.