Most systems today are going to be bonded by means of the racking system, if it is listed for it, which most systems are listed for it. Or with bonding washers such as WEEBS used within the racking system. Sections of the racking that are otherwise discontinuous, will then get bonded with your wire EGC. This means the EGC doesn't need to connect to every individual module. The racking system gets you the mechanical continuity as well as electrical continuity.
250.120(C) is what governs the size of an EGC that is external to a raceway, which you inevitably have to do, because racking systems don't come with conduit terminations or enclosed grounding lugs. This is why you commonly see #6 Cu as the size specified. It is not for the electrical capacity of the ground wire, but for being subject to physical damage. "Subject to physical damage" is a subjective concept, that will depend on your AHJ's interpretation, even though most of us may have a general idea of what it means,
Once inside a raceway, an EGC gets sized per NEC250.122, with the fuse/breaker being the input to the sizing algorithm, and if no fuse or breaker is applicable, you calculate what it would be if there were one, and use that size.
You might be thinking of the GEC as opposed to the EGC, where #8 is the minimum. The EGC (equipment grounding conductor) is for bonding all electrically inactive metal together above ground, and the GEC (grounding electrode conductor) is for making the connection between what is below ground (i.e. the grounding electrode) and what is above ground. It used to be that inverters always required a GEC, due to the fact that the inverters had a transformer inside that isolated the DC side from the AC side. Now that we have transformerless inverters, the standard AC-side EGC is all that is necessary to ground the inverter and also ground the array, because the DC-side is not a separately derived system like it used to be. You were able to combine your EGC and GEC, but it comes with a lot of strings attached to how you could do it, since the GEC needs to be irreversibly spliced, and had a minimum size of #8 Cu.
Now, it is a lot less common that you even need to think about GEC's in PV systems. You need them when you involve transformers, a separate service, or an inverter with transformer isolation. The typical system that only uses transformerless inverters, and connects to an established service already, doesn't need a GEC.