From what I can gather every follower gets an individual copy of every post sent to their server who either deduplicates it and stores it in their database or just stores the message per subscriber.
So if 10,000 people on server A subscribe to someone on server B then server B will send 10,000 duplicate messages to server A.
So if 10,000 people on server A subscribe to someone on server B then server B will send 10,000 duplicate messages to server A.
I hope I’m wrong but that’s what I’ve heard.