[1] https://www.knowyourmobile.com/user-guides/iphone-battery-si...
These are the same problem. Nominal voltage lowers as batteries age, as does the ability to maintain a voltage under load. The batteries were "fine" for the first year. A larger battery would have been able to keep voltage under load at an acceptable level even as it aged.
> I expect the battery on the 13 to be bigger because I assume (with no research, admittedly) that the SOC and screen both take a good bit more power than the equivalents on a 6S.
And yet the 6 had a larger battery (same screen size). Personally I find the 6S (and 6) to be very thin. They could have easily been a couple mm larger without the consumer noticing (Apple could have made the camera lens flush!).