Just go to your average big popular subreddit, check out all the text of all posts and comments they week. That's still a minuscule amount of data. A few megabytes when uncompressed.

And Lemmy won't get to that point of popularity and traffic for a very long time.

And even then, it's an easy problem to solve. Each instance creates a chunk of a day's data, sign it and share it on a bittorrent like protocol. Even nntp massively archaic infrastructure can manage this, it is a piece of cake for Lemmy to do.