Sized by the breaker, not the circuit conductors
The equipment grounding conductor is sized from the overcurrent device rating, because its job is to carry enough fault current to trip that device quickly. It is a safety path, not a load-carrying conductor, and it does not have to match the circuit conductors.
The upsizing rule
Where circuit conductors are increased in size — for voltage drop, most commonly — the equipment grounding conductor generally has to be increased proportionally. The reasoning is that a longer run has more impedance in the fault path, and the ground path needs to keep up so the breaker still clears the fault fast enough. This reference does not compute that adjustment.
Not the same as a grounding electrode conductor
This table is for equipment grounding conductors. Grounding electrode conductors — the ones running to ground rods, water pipe, or building steel — are sized from a different table based on the service conductors. The two are frequently confused.