In Tenga Toppa Gurren Lagann, there was a part where after they defeated the beastmen, Rossiu forced everyone to come up to the surface, even those who didn't want to. Never really fully understood why he did this. After all, if I recall correctly, Lord Genome already warned them what happens when the surface population of humans reaches 1 million. A census on those who live underground doesn't make sense to me. So why did he do this?
I would have to check, but I seem to remember that the trigger was 1 million total - it does not matter whether the humans are on the surface or underground, they still count. Keeping humans underground, and under constant attacks from beastmen, had the effect of capping that number.
And as far as I remember, the thinking was that people underground hampered the census efforts, thus preventing Rossiu from estimating correctly how far they were from the limit.