Activitypub does have many very small nodes, very few very large nodes, and few medium sized ones. A lot of this is people wanting to be on the same server that their friends are on (even if there aren't that many advantages to it), and just that structurally, running a largeish server is more cost- and time-effective per user.

It'll be interesting to (if that ever happens) see whether bluesky, once it actually becomes a decentralized system instead of a closed one, shakes out differently due to protocol differences, or broadly follows the same patterns.