Apple has been very proud of the iPhone 13 battery life improvements. For example, the iPhone 13 Pro Max boasts the longest battery life in any iPhone. Apple's battery performance comparisons to the iPhone 12 line-up shows that streaming video performance is what really sets the new iPhones apart.
iPhone 12 mini: 10 Hours
iPhone 12: 11 Hours
iPhone 12 Pro: 11 Hours
iPhone 12 Pro Max: 12 Hours
iPhone 13 Mini: 13 Hours
iPhone 13: 19 Hours
iPhone 13 Pro: 20 Hours
iPhone 13 Pro Max: 25 Hours
The iPhone 13 line features new low-power displays and a more efficient A15 chipset, larger batteries, more power-efficient components, and dramatic improvements in streaming performance, as shown by the numbers.
Apple claims that the iPhone 13 Pro Max and 13 Pro Max last 9 hours, respectively, when streaming video. This is in comparison to last year's equivalent models. The iPhone 13 mini's battery lasts 13 hours. This is more than the iPhone 12 Pro Max.
Apple clearly considers the computational requirements of video streaming services, such as Apple TV+ and, obliquely Fitness+, when calculating the power optimizations it has achieved through tight integration of its software and hardware.
Although battery life is dependent on use and other factors, actual results may vary. However, if these devices are anywhere near Apple's claims, binging your favorite shows should be easy with these new devices.
(H/T Jack for pointing it out!)