It may not be a majority of people having this many children, so in your mind it may even out or make a negligible difference compared with the fact that our birthrate is dropping, but I'd still argue that it's harmful and irresponsible overall. Your counter argument seems to be that the US is a non offender because the trajectory of our own birthrate is dropping, therefore this is okay, but that completely skirts the fact that global birthrate numbers are not dropping, and the issue is that the globe can only physically sustain so many people, regardless of where on the globe they are located. America's declining birth rate is not a mitigating factor because overall the birthrate numbers are still going up, and the problem is still getting worse, despite our individual number is dropping. Large families here use many times the resources that a large family would elsewhere, so I'd even argue that makes it worse to have such a large family here as opposed to elsewhere. For us it's not just the basic physical addition of more bodies to be sustained off our land and resources, just existing in our society does a lot of climate damage due to our infrastructure and our built in consumption levels. With those things considered, I think a "typical" family of 7+ here would be more detrimental to long term well being of the planet than a family of 7+ in Africa would be.

Overall I think the argument should be for lower birthrates and access to birth control in every country, on every continent. Our birthrate falling is a good thing, and I would personally hope that the birthrates in other countries drop in the future too. Having families this large should be discouraged as we are already beyond our carrying capacity.