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.
A lot of folks' fear is that when Mastodon grows it'll absorb bad people. I'm gonna say this. I think even if Mastodon literally grows to the size of current Twitter, the world will be better off overall just because of how Mastodon is structured and what values it represents and what tools it provides for dealing with those problems.