I can't understand why you are trying to refute the exact words of Wiz. Wiz said size is the most important factor, and small empires will be able to unlock Traditions faster. Between two equally small empires thus the other factors would kick, but as per Wiz words a smaller empire will be able to unlock Traditions faster than a larger one. Which is just silly regarding most of the Traditions.
Now if Wiz had said smaller empires will be able to generate Unity faster, but certain Traditions require other actions to be able to open the Tradition in question (get flagged for having conqured or purged etc for certain actions before you can spend Unity to open that Tradition), that would have been a different statement. But that is not what Wiz said, he spesifically said smaller Empires are able to open Traditions faster than larger empires.
Let's imagine this scenario, and begin with an assumptions:
Unity gain is affected by number of planets.
Empire A has 11 planets.
Empire B has 10 planets.
Now, are you saying that Empire B will definitely 100% unlock more Traditions than Empire A, i.e. there is no player agency or AI action on the part of B that could allow it to surpass A?
But that would be boring, so of course not - are we all agreed on this? So, I would speculate that other factors could into play.
In that case, the question is one about the quantitative effect of size vs whatever other factors contribute to Unity gain. Now, one might imagine that with a galaxy-spanning Empire it would be literally impossible to match the Unity gain of a single-planet one, assuming it is affected by technology and tech is equal. I don't see a problem with that, personally.
You do realise you are contradicting yourself in the above statements?
Wiz said certain Traditions can be used to off-set the malus large empires get to generating Unity. Ok, sounds good until you realise you first have to generate Unity to open the Tradition which off-sets the malus.
If smaller empires can directly go for the Traditions they please, larger ones first have to suffer the lowered Unity generation and finally be able to open the Tradition to off-set the malus Unity generation. How many Tradition will a smaller empire already have unlocked when the larger one is able to open the first meaningful Tradition (considering the first would go to buy off the malus to Unity generation)?
Unless the Unity generation boost for the Tradition off-setting the Unity generation malus is huge (it has to be worth the Unity cost for the Tradition so that Unity is generated at much increased rate to offset the Unity cost for the Tradtion) this will be a double-whammy against larger empires and then it will not be worth it to take it (it would be basically Unity points thrown down the drain). On the other hand, if the Unity generation is huge enough it will be an absolute no-brainer Tradition and everybody will have to go for it first. If everybody goes same railroad anyway (they never take the Tradition or they always take the Tradition), where is the point of such a system for a game?
What is your point, exactly? Oh no - large empires cannot get as many stacking buffs as small empires? Oh well, I'll just roll over everyone with my ridiculous fleet. Again, this is a question about the quantitative effect of Traditions. Which we don't know. Unless we imagine them to be extremely overpowered, there does not seem to be a problem?
Edit: Might as well expand with a hypothetical scenario using whatever-Tradition that reduces malus from size. Let's call this
Expansionism.
Imagine that Unity is generated at +1 / month. Suppose each planet after the home planet decreased this by 0.2 / month.
Now, we can unlock
Expansionism with 50 Unity points, and this reduces the malus by 0.1 / month / planet. By the time we colonise a mere 5 planets this gives a net gain of 0.5 / month over another Empire that has not taken
Expansionism, which then pays for itself within 100 months, 200 if we take the worst-case assumption that Tradition cost increases by 100% of base each time. Now, at the other end of the spectrum, if it takes 300 years to see any net return, then of course Expansionism is a waste of Unity points. Hopefully the balance is well thought-out.