they have a really close physical size to each other and even if it had more components they could've easily fit a 4000 mah battery " SAFELY " , the galaxy s8+ is a fantastic device but if consumers dont push companies to use bigger batteries then we will never get better battery life they will if they can...
the space for battery is really limited since they need to include all extra flagship's feature (NFC, MST, DAC, heatpipe, barometer, iris scanner, bigger main camera with OIS, rear FPscanner, heart-rate sensor, wireless charging, etc)..
It's funny that most people are pretty much proud of having the bragging rights when they are actually so backward in practical life.
And as per the stats, 4-inch smartphones no longer belong to the midrange segment, but low-end segment, as most Android users prefer big phones.
Everything as from the midrange segment is phablet these days, with 5.5 inches being the sweet spot.