While big families are a global negative, they're a national positive because of the aging population and fact that there will be no state pension or free at the point of use universal healthcare if the number of working age members of the population per retirement age person declines below a certain point.
The average number of children per woman (in her lifetime) in the UK is between 1.5 and 1.65, which is below the replacement level - on a global scale its 2.4, and over 4 in some countries.
Its a catch 22 on a national/ global scale. Individual countries need to replenish their working age population to support their increasing elderly population (as they can't ethically reduce life expectancy!) but the world can't support an ever expanding population with a global birthrate above the replacement rate obviously...
So you could talk about that.
I have 3 kids. I do think going on and on having children without thinking about the consequences is irresponsible and selfish. A friend of mine says she's bringing up her children to contribute to the world in a positive way so they'll be an asset, which is both admirable and also obviously arrogant (she admits this) and obviously nobody can guarantee that...
Its not quite as simple as having more than two children being all out selfish and bad, and I do think having any at all somewhat abdicates the right to preach too righteously, especially combined with things like a high consumption lifestyle, heavy shopping habit, bigger than necessary centrally heated home (or more than one home) lots of travel, lots of big pets and several big vehicles to transport pets in etc. It might not be as bad as the long term impact of another child but multiple large pets and vehicles and buying a lot of "stuff" you don't use (including clothes) is equally selfish and cognitively dissonant...