The 16GB version of the Pi 5 includes the revised "d0" stepping of the Pi 5's BCM2712 processor. For the Pi's purposes, this chip is functionally identical to the original version but uses slightly less power and runs slightly cooler because it cuts out silicon used for features that the Pi 5 didn't take advantage of.
Raspberry Pi CEO Eben Upton writes that the 16GB version of the Pi 5 is possible because of other tweaks made to the d0 stepping of the Pi 5's processor, plus an updated LPDDR4X chip from Micron that could fit eight 16 Gbit RAM dies inside a single package that could fit on the Pi 5's board.
