A small amount of high-grade bonds with low default risks are transacted at organized exchanges. Organized exchanges usually impose a volume limit on corporate bond transaction. Large volume is all done through OTC. Take the U.S. market, a highly developed bond market, as an example. Institutional investors hold about 90% of outstanding bond and individual investors hold the balance.
These very wealthy individuals, more like institutional investors in nature, usually hold high-grade corporate debt. A very small amount of low-grade bond is in the hand of individual investors because bonds are downgraded as a result of declining corporate performance. From a logical point of view, QIBs and OTC are the defining features of corporate bond market.