I like the idea. In regards to lemmy and its instances (so I guess a smaller Scale to what your suggesting … Though I still don’t fully get the fediverse).
I did something similar when picking a Lemmy instance. What was the instance that had defederated others the least. There was a git page somewhere that listed them all.
On the flip side I’ve heard a notion that no one single instance should become the “main” instance. Probably with your suggestion new users would gravitate to the big circle. Thus making it the “default”. But maybe it’s different when looking outside of lemmy instances to the wider fediverse.