Let's say there's this player that has the build and countenance of Shrek. He's a big dude. So big that his coach tells him not to line up for free throws to save him a trip down the court thus preventing cardiac arrest.

The question is, why is it that Shrek the ogre is allowed to use his massive girth to shove his way into the paint but the person defending him isn't allowed to use an arm to brace against it?

Purely hypothetical situation here.