One of the broad assumptions behind recessions and recoveries is that during the “boom”, excess capacity is built into the system as manufacturers & service providers expand to meet increasing needs (today, and in the future). During the recession, manufacturers & service providers pare back, leaving capacity idle.
Part of the reason that the recovery (typically) gains steam is that bringing back this idle capacity (both in physical and human capital) is cheaper than building (or training) new, and it allows the economy to “roar” back into high gear. In some high level sense WW2 leveraged all of the physical and human capital that was idled by the great depression; while huge plants were built and millions of workers mobilized much of the initial lift was caused by leveraging what we had that was unused at the time.
When I look at this “boom” and recession, however, from the point of view of the USA, it doesn’t seem that we over-invested in productive capacity. Much of the investment was in residential real estate and commercial real estate for distribution, retail and services.