CMOS Inverter Voltage Transfer Function
The voltage shifts because additional over drive voltage is required to generate additional \$ I_{ds} \$ to match the \$ I_{ds} \$ of the PMOS.  This arises because of the reduction of W reduces the \$g_m\$ of the NMOS transistor.

The curve will have it's roughly sigmoid shape, which is dictated by how the transistors transition from triode to full active, but they will not be perfectly symmetrical. \$V_{th} \$ , \$g_m\$ are very unlikely to be perfectly matched.