The diagonal screen measurement is meaningless because of improved screen-to-body ratios. The iPhone 12 Mini is smaller than the 4.7" iPhone 6/7/8, despite having a significantly bigger screen.
Iphones always had screens that were close to the sides which is what really matters. The chin and forehead don't matter for reachability because you never had to reach them.
They don't matter for reachability but that's only one metric. They certainly matter for, say, whether or not it'll fit in your pocket.