Grounding rod conductor dimensions for 200amp service

Hi, I'm trying to understand Section 250.66 of the NEC 2014 (National Electrical Code) document for how to connect the ground of an electrical service to the electrode system. So basically it says in Table 250.66 if I'm using 4/0 Aluminum wire to supply my 200Amp service and there is is a feeder between the Service Disconnect and the Breaker Panel that is 20 feet away (i'm using my case for an example), then the ground conductor in that bundle needs to be AWG #2 size (aluminum). If I'm connecting the Service Disconnect panel ground bus to a grounding electrode system (ground rod or grounded metal water main), then that conductor never has to be thicker than AWG #6 Copper per 250.66.A. Am I understanding this right?