Well actually... Japan has a bit of problem, the old will soon outnumber the young.
No strictly saying that population *has* to grow, but it normally has for all of human history (minus a few black swans) and our economic models and ability to care for the elderly has generally relied on that fact. A human ponzi scheme if you will.
There are obvious potential benefits to lower population (basically resources as you said). But some major economic and societal adaptions would have to occur to prevent some seriously bad consequences.
Actually, it looks like we (at least the youngest of us) are going to find out. Many economically advanced countries already experience negative population growth due to low birthrate, and the ones that don't require immigration to keep growing.
Of all the major economies, US might be least impacted due to immigration (Japan for example doesn't really allow significant immigration).
Right now, low birthrate in the developed countries can be offset by immigration. But global population will go inverted (ie decline) in the 2080's according to current projections. It's expected to peak around 10B which isn't much more than what we have today (8B).
As countries advance economically people tend to favor birth control and are less inclined to reproduce until later in life, or not at all. Global population *growth* actually peaked in the 1960's and is plummeting for negative terriritory as we speak.
https://en.wikipedia.org/wiki/Projec...2_revision.png