Apple just out spec’d the Phone’s cpu, camera, screen, speakers, etc... compared to what the battery could power. They didn’t do this with the 5S or earlier, but they did it with the 6(or is it the 6s) or later.
Couple that with Apple’s lust for thinner and thinner phones, sacrificing battery volume to meet that goal.
Don’t believe me? Look at Apple’s actions as proof.
In many ways, putting in an inferior battery is the right business decision to sabotage a device.
99% of iDevice users would be clueless when you talk to them about battery size, peak power and current. Since Apple knows everything that goes into the device, they knew they can cheap out on the battery and nobody would notice.
It worked out for many years.