Howard Oakley writing at The Electric Light Company on how Apple silicon has grown in the move to the M2 family:
Apple silicon CPU cores are grouped in fours, forming a cluster run at the same frequency and sharing cache. M1 Pro and Max chips are unusual in only having half a cluster of Efficiency (E) cores, for which their frequency is managed differently from the full cluster in a base M1 chip. This ensures that background tasks are completed no slower on their two E cores than they would be on a base M1 chip with twice that number.
Increasing the total number of cores for the M2 Pro and Max shouldn’t have been a difficult design choice. Adding more Performance (P) cores would have required a third cluster, greatly increased energy usage and heat production, and resulted in a larger and more expensive chip. E cores are smaller, more frugal in their energy use, and produce less heat. Adding two E cores was probably the least Apple could do to improve the performance of the M2 Pro/Max over those M1 variants.