When discussing such questions, we need to be careful to distinguish the following:

1. Is a world containing population B better than a world containing population A?
2. If a world with population A already existed, would it be moral to turn it into a world with population B?
3. If Omega offered me a choice between a world with population A and a world with population B, and I had to choose one of them, knowing that I'd live somewhere in the world, but not who I'd be, would I choose population B?

I am inclined to give different answers to these questions. Similarly for Parfit's repugnant conclusion; the exact phrasing of the question could lead to different answers.

Another issue is background populations, which turn out to matter enormously for average utilitarianism. Suppose the world already contains a very large number of people wth average utility 10 (off in distant galaxies say) and call this population C. Then the combination of B+C has lower average utility than A+C, and gets a clear negative answer on all the questions, so matching your intuition.

I suspect that this is the situation we're actually in: a large, maybe infinite, population elsewhere that we can't do anything about, and whose average utility is unknown. In that case, it is unclear whether average utilitarianism tells us to increase or decrease the Earth's population, and we can't make a judgement one way or another.