No, population growth slowing down has nothing to do with a collective agreement on their being too many people.
It's simply that people in developed countries don't want to deal with the trouble of raising more than 1 or 2 kids because raising kids is difficult and expensive. You need 2 kids to replace the 2 adults, but then you add in other mortality factors and it drops below replacement.
Most of those people aren't agreeing that there are too many people, hell in cases like Japan, even their governments want them to have more children, they're just focusing on what strikes a balance for their comfort.
It's simply that people in developed countries don't want to deal with the trouble of raising more than 1 or 2 kids because raising kids is difficult and expensive. You need 2 kids to replace the 2 adults, but then you add in other mortality factors and it drops below replacement.
Most of those people aren't agreeing that there are too many people, hell in cases like Japan, even their governments want them to have more children, they're just focusing on what strikes a balance for their comfort.