XMPP is featherweight compared to Matrix because it does less things than Matrix.

Having a gigantic group chat with hundreads of people, with encryption, history and media synchronized between everyone without relying on a centralized server is something that XMPP doesn't even try to solve. Matrix does.

Prosody is a great project, I used to run my own Prosody server before moving on to Matrix. But people shouldn't lose perspective of the state of the protocol.

XMPP is okay if you plan to chat with a few people individually that are willing to spend time choising the right client with the right plugins and if you don't care about modern features but anything beyond than that and it becomes a huge headache.