excluding IPO proceeds, existing public companies in the US raise about $200B a year through selling shares on stock markets. This DOES NOT include stock-based comp, which is simply another form of funding operations post-IPO using public markets.

Stock based comp is another $350B a year in US markets alone. So if you think about public markets as an avenue for companies to raise capital, post-IPO firms are doing it to the tune of more than half a trillion a year.