Chip size and thinner?
Nothing much to do with chip size, more to do with skinnier screen assemblies, thinner batteries. Even Apple's excuse about the 3.5mm jack socket is a lie.
Really in terms of battery and loudspeaker (if not holding it to ear, and I see LOADS of people using their phones ST-TOS style) and being robust the phones need to be fatter. Even 10mm is fine. Less than 8.5mm is a fashion statement, not sensible. People want better battery life and more robust, not thinner. No-one should need buy the stupid bumper covers, that's an indication of design failure. I've never used one, though I got one included with last phone.
Also the chip shrinkage rarely ever affects THICKNESS, but the PCB area. The Samsung SC6400 family ARM (variant in original iPhone) allowed easy design with no external PCB traces to Flash memory and RAM, as it had the RAM and Flash chips inside the package, piggy-backed. Not possible on Intel in even 2008. Even that design would let you make a 6mm thick phone.
AMOLED saves some thickness as there is no polariser, no rear/edge diffuser/lightpipe/LED illumination arrangement and the actual bare panel is skinnier than an LCD. Shorter life though as it's not LEDs in the conventional sense but more like an EL panel.