Every system has a capacity of some kind. It must if it operates with mass/energy. There are limits to everything. There are limits to how tall we can actually grow, there are limits to how much stress we can sustain and there are limits to how much we can know at any one time. Look at any system long enough, and you will find an upper and lower bound (the limits) of its operation. For biological systems, there is also a range that is the most efficient (long distance runners know this as a marathon pace).
In terms of the economy of the United States, the limits to growth have historically been based on population (workers available), raw resources and demand (both internal and world demand).