Physical design is outside my area of expertise, but I understand that a large part of the increase may be due to improved fab. 15-20% speed improvement per node would account for the majority of the clock speed gain. Also, with smaller feature sizes comes the ability to trade away some of the density for further increases in the frequency, without actually requiring more transistors.
The ones in Sapphire Rapids run 2x+ faster. Likely there is more pipelining and many more transistors used to reach those clock speeds.