http://tweedge32j4ib2hrj57l676twj2rwedkkkbr57xcz5z73vpkolws6vid.onion/2022/request-amplification-in-mastodon
Essentially, the problem is that Mastodon wants to generate link previews for any link you put in a post, but all Mastodon servers will generate link previews independently of one another - they don’t share the link preview itself, just the link. This means that for every post you make, both the server you’re on and any server receiving your post will access the sites you’ve linked.