These numbers are misleading and need more unpacking.
When Vatican claims "the number of Catholics globally rose 1 percent, to 1.3 billion, in 2015" it continues to include RCs who have left the institution.  But, according to the Vatican, once a Catholic, always a Catholic.  So, we'll count 'em no matter where they are.
As for the surge in vocations in Africa especially - in poorer countries as formerly in Europe/America joining the clergy was an upward career move, especially when they were few options for poor males.
An increase in clergy has less to do with religion than with economics.
But, as long as the RCC insists on no contraceptives, they'll give Muslims a run for higher birthrates, at least among poorer folks who will be urged to procreate, procreate, procreate - or you'll go to hell.  Muslims & RCs are leading contenders for high birth rates.  Question remains, who will become more first in sex education?