Here's the answer

April 25, 2020   photos  

To make contact at first, when there is a short circuit, the short circuit current flows towards earth through earth pin. In addition to this, the BIG short circuit current flows through the earth, so the earth circuit has to be with a big maximal current. That's why the earth pin is thicker! the earth pin longer means earth contact first to instrument to avoiding human electric shock. If short circuit current will flow very high so earth pin size is double.