I don't have the link to the US census bureau in front of me, but I think as of 2018 more than 50% of employees worked for firms with >500 employees.

And, of course, there's nothing preventing a small/medium business from incorporating, either. "Corporation means big, small business is a different thing" is common shorthand but not actually how it goes.

The vast majority of people working for small companies do not earn much. A few doctor firms, and high end legal/engineering firms maybe, but most employees of smaller businesses lose out on total comp to big businesses, and government.