Anyone who thinks the population should be reduced could easily lead by example. At least they'd be doing something about it instead of just whining.
But seriously: how would we actually reduce the population size? Sterilization? And would those advocating for population reduction be the first in line? I highly doubt it. For that matter, would anyone volunteer?
If no one did volunteer, who would be the arbiter of who gets to reproduce and who doesn't ... and upon what authority would they act? Historically speaking, the population reduction crowd doesn't have a very good track record when it comes to active members.
It's a slippery slope - unless you don't mind being thought of as another Francis Galton ... or worse. Like many other so-called "solutions", population reduction might seem like a good idea ... until you actually think about it.