You can't really compare die sizes of a MCM and a single die and expect to get transistor counts out of that. So much of the area of the MCM is taken up by all the separate phys to communicate between the chiplets and the I/O die, and the I/O die itself is on GF14nm (about equivalent to Intel 22nm) last time I checked, not a new competitive logic node.

There's probably a few more gates still on the AMD side, but it's not the half again larger that you'd expect looking at area alone.