Ubi, you CAN’T know, because unless some other factor balances the numbers, we’re in trouble. If there are a lot more girls than boys, there MUST be girls left over.
Obviously, your set of facts, that there were 1) a gap 2) large families (population growth) and, 3) no surplus of girls, is wrong because it’s not mathematically correct.
Once you get the right set of facts, you might realize which is the cart and which is the horse.