hi!
unsymmetric input diff pairs cause mismatch to your diff stage. if you notice you have a "tail" to control the current flowing through both pairs, ideally you have identical sizes so same currents flow through them. since current is proportional to W/L in a mosfet, diff W/L ratios mean different current through your transistors. usually gain is equal to gmRo where gm is transconductance and Ro is inversely proportional to current Ids. when you change W/L, you change current and you change Ro hence the gain also changes. usually gain is inversely proportional to current.
- al