RAM size is covered under “faster”.
6s over 6: Faster, better camera, water resistance, force touch
7 over 6s: Faster, better camera, no headphone jack
8 over 7: Faster, better camera, wireless charging, tru-tone display
These are all minor incremental changes, the sort of thing you normally see in an S revision. The phones all look nearly identical; the buttons shifted locations slightly and the headphone jack was removed so 6/6s cases won’t work on 7/8, but at a glance you could not tell the four “generations” apart.
Now, does the case necessarily need a radical redesign to be considered a new generation? No, certainly not. But they could have shrunk the bezels to match the 2016 android status quo, certainly.
And you can make the argument that taking all the S-class improvements over the past 4 revisions, the 8 is a substantially better phone than the 6 and worthy of being considered a single generational improvement. It’s vastly faster, water resistant, has force touch (not that anyone cares), wireless charging, tru-tone display, and unfortunately no headphone jack. All those things taken together make a compelling upgrade from a 4 year old iphone 6.
But from the 6s, much less so. I have a 6s and I see no particular reason to upgrade to an 8. I’ll get an X, though.