Being against "overpopulation" isn't necessarily a fascistic thing but:

1)-The world is never going to have 30 billion people, most models predict that we'll hit 12-13 billion and then decline from there on.

2)A lot of takes about overpopulation ignore that a big reason behind the growing population is the lack of access to quality family planning and sexual education in developing countries. This combined with how there are still some countries in which having multiple children is basically necessary if you are poor drives population up. Takes on overpopulation that account for these facts and aim to solve it by helping the developing world aren't fascistic, however simply ignoring these factors can lead to people being racists against those from developing countries for "breeding like rabbits" etc.

3)A lot of overpopulation fearmongering comes from Malthusian ideas of the 20th century, which leftists are of course suspicious of

4)Those in developed countries on average consume way more than than those in developing countries, and a lot of food in the first world goes to waste, so an economic system that distributes resources with less waste and with more focus on human dignity could make it so that we could be using less resources while having higher a population.