Yeah let's see
India: 17% of world population, 1.58% Catholic
China: 20% of world population, 0.75% Catholic
Indonesia: 3.5% of world population, 3.05% Catholic
On the other hand, if we look at highest fertility rates in the world:
Niger: Fertility rate 7.46, Catholics 0.1%
Mali: Fertility rate 7.42, Catholics 1.54%
Somalia: Fertility rate 6.76, Catholics n/a
Okay, well maybe if we look at the most Catholic nations...
Brazil - most catholics in the world at 136 million, fertility rate 1.91 (below replacement)
Portugal - highest percentage of catholics outside the Vatican at 94%, fertility rate 1.47 (dangerously low)
Get a new scapegoat guys.