The plot shows the relationship between the number of users each instance in mastodon had two weeks ago (horizontal axis) versus the number of new users in the last two weeks (vertical axis). Each point correspond to a instance. We can see that there is a relationship between them, meaning that the "rich-get-richer". However, the most rapidly-growing instances are Mastodon.world and mas.to which are not the biggest.
https://sb-82bx3xp8al.b-cdn.net/media_attachments/files/109/383/878/037/428/740/original/4ce8185f8a047933.png