To be fair Iran has 1+ trillion USD economy while Saudi doesn't (yet, projected to get there by end of this year). So it has more economic weight in an economic group. All of the NG8 are 1+ trillion USD economies... Or were, until Turkish Lira devaluated so much it fell below $1 trillion.
But then the political meaning is questionable. if US treaty allies like Turkey and Mexico can be invited then what is the point? Or hostile states like India for that matter?
If it's just "collection of strong economies outside G8" then might as well invite South Korea.
Iran's economy is (nominal) about $200 billion. At PPP figures it is $1 trillion.
Saudi Arabia's (nominal) is $700 billion.
At PPP figures it is $1.74 trillion. Also don't forget that half the world depends on its oil.
As you can see, this isn't about economics but about Russia's attempts for a power play in the region. If it was truly about economics, either Iran + Saudi Arabia are invited or only Saudi Arabia.
This isn't me about being a SA fanboy but on China's diplomatic priorities and commitments to the region. As for SCO, I don't rate that highly Iran's inclusion in it. In my opinion Iran is merely the first step before getting (possibly in this decade) the real prize, Saudi Arabia + other Gulf States.
As for Mexico and Turkey, I am ok with them. Not everyone can be China friendly and the group would need to demonstrate acceptance of different countries in order to prove that it is a true global south multilateral grouping. If not, you might as well as call it Russia's + China's friends group and move on..
That's probably how Chinese officials will evaluate this new idea from Russia and then promptly move to shut it down. As it stands, imo this proposal isn't in China's interests.