Sp3r4z's stats page also has some stats regarding centralisation:
It lists the Gini index in regards to users on Mastodon instances. The Gini index is used in economics to measure income inequality, where a 0 would be perfect equality and 1 complete inequality (all income in the hands of one person).
The Mastodon federation Gini index in regards to users per instance is 0.96.
"The 3 biggest instances representing 52.26% of total users"