what is overdrive voltage
When you increase the overdrive voltage by increasing the current, your gain decreases because Rds of the transistors decrease proportionally with I while gm increases proportionally with sqrt(I). To increase Rds once more, you can increase the transistors' channel length which increases Rds once more (but after certain length Rds increase becomes negligible), while increasing W with same value, thus keeping W/L the same. This will result in Veff increase with sqrt(I) without significant loss in gain, also mismatch between transistors' will improve due to area increase (according to Pelgrom's model).