I don't think siding with the most powerful is the motivation for the US. I think the US sides with countries that it can benefit from in some way. Which makes sense.
You basically answered your own question right there.
Influential states are almost almost more useful for a powerful country in this case a hegemon like the US, than lesser powerful states.
Except maybe the Arabs in Iraq, Syria, Lebanon, and Yemen. Are you still so insistent that all Arabs think alike? Do you not think it is possible that two Arabs, even in the same country, might have different political leanings?
People are complex. You can't categorize them so easily.
I was talking in general. There can be no doubt that most of the 450 million Arabs would side with KSA just like they would side with every other Arab country against Iran or any other foreign country for that matter. That's completely normal.
We saw that most recently when the US invaded Iraq in 2003. I don't think that there was a single Arab country where most of the locals supported the US invasion and not the Iraqi people.
I doubt that Syrians, 80% of them being Sunni Arabs with ancestral, tribal, religious, linguistic, historical etc. ties to KSA, would side with Iran and especially not after Iran's role in Syria. Moreover most Syrians are Arab nationalists. Why should they side against other Arabs and another Arab country in return for siding with non-Arabs and a non-Arab country?
Same story with Lebanon, which has a sizable Sunni Arab minority, with the exception of Hezbollah which is basically an Iranian proxy.
The example with Yemen is even more clear. Only the Houthi's would be an exception. Why would for instance the Zaydi's of Yemen (around 40% of Yemen's population) go to war with their identical Zaydi brethren across the border in Najran?
Same story with Iraq. The Shia and Sunni Arabs in Southern Iraq are basically identical to those in nearby KSA. Same tribes, clans etc.
But yeah, the Iranian controlled Shia groups in Southern Iraq would probably side with the Supreme Leader but we are talking about a few 1000 people at most.
Anyway an Iranian attack on KSA would be perceived as an attack on Arabs and the Arab world in the current state of affairs. It would also have religious implications.
Also you need to forget the rulers here (House of Saud, Mullah's etc.) and look at the views of ordinary people.
When Arabs complain/criticize each other it's mostly aimed at our mostly unelected regimes. There is hardly ever any real personal animosity involved. I can only think of the really sectarian lot and their hatred is religiously motived. They will hate the "wrong Arab" as much as the "wrong Polynesian".
Let's not even talk about ACTUAL politics. How many Arab countries would ACTIVELY support Iran against KSA, should a war break out? I doubt that a single Arab country would do it.
How many Arab countries are siding with the Houthi's against KSA for instance? The answer is zero. None. Nada.