if the birth rate had stayed the same since Malthus's time.

That makes no sense to argue: Malthus's time was part of the demographic transition. Of course I would agree that if the demographic transition continued post-Malthus - as it did - we would see higher per capita (as we did).

But look up the extremely high birth rates of some times and places (you can borrow some figures from http://www.marathon.uwc.edu/geography/demotrans/demtran.htm ), apply modern United States & Western Europe infant and child mortality rates, and tell me whether the population growth rate is merely much higher than the real economic growth rates of ~2% or extraordinarily higher. You may find it educational.