Why do we prefer to size the return air duct at lower velocity as compared to the supply air duct for same AHU

There are few fundamental properties that we often use interchangeable but in its true sense they are not synonymous, we have to first recognize that duct static pressure at design airflow rates is not the same thing as fan static pressure. Fan static pressure define in ACGIH handbook, excerpt below:

Below is an illustration of the inter-relationship between fan total pressure, fan velocity pressure and fan static pressure, which is self explanatory.

Working out the example below which will provide you with a better understanding of the dynamics of what is going on in your ducting.

So simply

FSP = SPoutlet – SPinlet – VPinlet

a larger suction duct has lower velocity then VPinlet will be lower, hence we will have “more” fan static pressure available for duct resistance.

Here is a link to an article that describe the salient difference between the use of total pressure and static pressure to select your fan, Fan Static Pressure or Fan Static Pressure: Which Is Correct When Solving Ventilation Problems?