You're misunderstanding the misunderstanding.
You're right! The relative sizing of models is dead simple. This doesn't help when there are improvements between release numbers, and models are released one at a time instead of as a family.
When Sol 6.4 comes out and the latest Astra is still 6.1, which do you use?
I use Sol 6.4 because it's newer (and "big") unless there is a specific benchmark that 6.1 Astra scores better on that I think I need.
Or I might try both and pick whichever I prefer for a given task...
Okay, now how many generations until you use Terra over Astra?
Probably never? Unless they get super far out of sync, a "medium" model just isn't the right choice for a "large" (or "extra large") model task.