Symmetric Inverter Sizing Based on load capacitance and propagation delay
Mobility of holes is smaller than mobility of electrons. Thus the same width nmos transistor can have more current flow than the pmos. In order to have balanced circuits in terms of high-to-low and low-to-high delays you need to have pmos width larger than nmos so that the W/L ratio in current equation of pmos increases until the current of nmos and pmos are same resulting same high-to-low and low-to-high delays. Practically, the width of pmos must be close to two times the width of nmos.