Most certainly a compromise. More inputs amortizes more of the overhead (power pins, input pads etc), but too many wastes density. A similar issues shows up i

Most certainly a compromise. More inputs amortizes more of the overhead (power pins, input pads etc), but too many wastes density. A similar issues shows up in FPGA where four-input lookup tables (LUT4) were the sweet-spot for many generations but in the past few years LUT6 (and hybrids like the ALM ~ 2xLUT6) have taken over.