I think this is likely to be a sleight of hand analysis on the HuffPo columnist's part. Firms with fewer than ten employees, which the column alludes to when talking about small businesses, make up just 5% of employer firms. Census doesn't define "small business", and SBA defines it in the context of specific verticals, so that a small manufacturer might have 500 employees while a small law firm might have far fewer. Whatever the case, firms with 500+ employees represent by far the largest segment of all employer firms, by just about 3x the number of employees of any other bracket.

(Caveat: I may be misreading some of this.)