Probably a stupid duct size question

Ok, so unit installation tomorrow. Have the new unit sitting here, 4ton ac/gas package.

Looking at it and the discharge and return square to round flanges it came with are 14” on the round side and pretty much take up the whole square. The discharge/intake on the unit are about 15” square so there really isn’t much room for a bigger size.

From my understanding a 4 ton requires an 18-20” duct size for proper airflow.

Am I overlooking something? Why would these come with 14” from the factory?