its literally a catch 22 situation, its a matter of what perspective (regarding the time factor) one is looking at.

Over population will cause pressure on the sustainability of resources on the planet, not to mention inefficient use of resources and resulting pollution.

Under population in a society will put other pressures on the sustainability of a society - more of the economic type and on a longer time frame. The pressures would be more about having a critical enough market size that can support enough jobs and hence a decent enough quality of life.