Who said this about the screws?

Exactly what code are you referring to? Or are you just going by what someone told you?

It is not the size of the screw that is a problem, it is that if the screw is not drawn up adequately tight it will not be a long term reliable connection.

If you try and draw a yoke mounting screw up tight enough with plaster ears on drywall you will not maintain adequate contact as the drywall crushes.

The ground connection has to be gas tight to have long term reliability, and that requires a pretty high torque to load the screw threads enough.