People were worried when Mastodon went from 20,000 to 500,000 people too. I don't see why you would think 1mil -> 2mil to be any different. I really don't agree with the comments that suggest that Mastodon shouldn't grow.
@Gargron No one seems to really recognise that part of the great thing about Mastodon is that you can pick an instance as per your tastes, moderated as such. The issue with Twitter having bad people is negated on Mastodon by blocking accounts and servers; you aren't trusting only one central moderation team.