I would say call that tangential. Properly functioning batteries that are undersized are a battery life problem. A malfunctioning battery that cannot deliver enough current for the CPU at full power is a different problem. It could be argued that the smaller battery would degrade faster due to more cycles, but I don't recall whether there was general dissatisfaction with battery life on the 6S or not. 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.