There is no such thing as a natural or healthy limit
per se; who or what should decide what that limit is ?
You are right to say an equilibrium state will be reached.. but that is only because the resources become more and more depleted. When an organism first expands into a new area (of foods/light/recources) and
it is given the chance, it will consume like crazy. The "given a chance" part is crucial, there are no organisms that use some sort of birth control. Well, apart from humans, and we fail miserably. Instead of birth regulation, nature uses death to control group-size.
Selfdestructive growth is very common in nature, the difference is that we are just very good at it.
Last summer, my gardenpond became infected with an agressive algea. It took over the whole pond in a few weeks. Then, after it grew into a large green blob and used up all nutrients, it suffocated everything.. including itself. A few weeks later still, my nice pond was reduced to black smelly water of death.
This spring I'll use the water from the pond to feed the plants in my backyard.
There is life to be found in death.
Here's a bit about population growth. The smooth leveling off towards a steady state is just one of several possible systems.
http://www.nas.nasa.gov/About/Education/Ozone/modeling.html
I still think colonizing other planets is a good option in the long haul. Who am I to forbid a couple from India or south Dakota from raising 3 children when both have good enough living standards ?
For now, we should just try to think of ways of sustainable lifestiles into the future. Restrictive measurements are not the way to go.