A 5.15-inch display area at an 1125 x 2436 ratio would actually be slightly narrower physically than the iPhone 7's display, so content would appear slightly smaller on the screen at around 174 points per inch rather than the 163 points per inch of the iPhone 7.)That sounds sub-optimal... default text size is already pretty small on the 4.7" devices; smaller, I believe, than on the 4" or 5.5" devices. (I personally use a 5.5" phone in "Zoom" mode... and I have good eyesight.)
They made font thicker and bolder in iOS11. It actually looks pretty poor imo, but it should help with people who have issues reading text on iOS.