I like this subject. Both relating to too much, and too few people...

First, regarding absolute human numbers, we do have too much, mostly because we are rapidly using up fossil resources (not only fuel, but also fertilizers), and some important ones already peaked (phosphorous for example).

But I don't believe that making your population decline fixes things, it only create other problems, the solution for the problem that I mentioned last, is develop more technology.

Malthus was right on his diagnostic, wrong in the solution (please, stop blabbering that Malthus is some stupid dude that is wrong, even on school I heard that, more than once, by several different teachers in several different grades), Malthus was absolutely correct in his calculations, he only did not accounted for the mining of phosphorous (that is one of the major enablers of green revolution).

Now we are running out of phosphorous and energy, and our food will return to levels Malthus calculated, unless we figure new tech, I hope someone does so soon.

Now regarding declining fertility levels... This is a problem too.

In history, there are several accounts of prosperous civilizations that had declining fertility, and all of them collapsed (usually when fertility reached around 1.3 per women). Several of the current western world problems are related to that, the need of external wars to keep some income flow (alright, no major powers are at war... between themselves, but how much wars US, France, etc... are involved with other smaller countries and forces? Even Brazil that is not developed is currently participating in a occupation force in Haiti and in civil war, although the media refuse to count the latter), debt problems, specially coming from welfare programs and wars, economies that are following their population numbers (thus we have Japan for example with economy declining with the same speed that their population is), and a increase in far right and far left party powers (example: brazillian military dictatorship apologists never been stronger, Greek far-right went from having no votes in parliament to 7%, and a estimated 35% in next election, Japan also has a new far-right nationalist party that got some respectable votes, the rise of Tea Party in US and Democrats steering leftward...)

And some groups, know of all this, and have a plan to take over when they can... The most famous one being Al Quaeda (that is the Arab translation of the name of Isaac Asimov books The Foundation, and it was known that Bin Laden was fan of the series). Al Quaeda is clearly aiming to teach their followers to keep a high fertility rate, so that they can conquer developed nations when they collapse under their own bureaucracy and population decline.

All those advocating that to fix the first problem I mentioned (peek resources), don't realize that by doing that, other nations still have population increasing, some of them will collapse because of it and cause further problems for everyone (and most likely will ignore the attempts of developed nations to stop their own population decline), and other nations are increasing in population on purpose, knowing well of its military effects, and will use it against any fool that voluntarily decline his population and ends struggling to keep itself up.