Kr = 1; Kr = Kn/Kp. Kn = UnCox(W/L)n, Kp = UpCox(W/L)p. (W/L)p = 2.5 (W/L)p. This is the characteristic for a symmetrical inverter. If Kr < 1 then the fall time will be slower. If Kr = 1, then it is ideal, if Kr < 1 then it is faster.
It is because of the mobility of the electrons and holes which are different. The mobility of electron is 580 cm2/Vs and holes mobility is 230 cm2/Vs. If you derive from the above equation, it is about 2.5 times.
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Because of the (W/L) which is the width over the length factor. You are changing P MOS width rite? But the Kr changes. It will change the voltage transfer characteristic of the inverter. From there only,you can see the fall time and rise time.