I ran an instance for a bit when I thought the migration would be more substantial. Basically, each instance gets a full comment of every comment and post, less the heavier data like images and videos (but maybe sometimes images).
Its also all publicly available. If meta wants it, its all out there in the open now, no federation required.
So yeah, it could become an issue for smaller instances, but honestly, its a problem that smaller instances would likely welcome. I’m by no means advocating for meta here, but content, submissions, engagement; they are the life blood of places like this and we just do not have enough. No parts of the fediverse do.