> Arguably a more efficient approach might just be to have a tax that adds on to corporate tax incrementally for every % of market share a company has above say 7-8%.

How is this more efficient? You'd still be applying all of the inefficient regulatory rules intended to mitigate a lack of competition to the smaller companies trying to sustain a competitive market, and those rules are much more deleterious for smaller entities than higher tax rates.

If you have $100M in fixed regulatory overhead for a larger company with $10B in profit, it's only equivalent to a 1% tax. The same $100M for a smaller company with $50M in profit is a 200% tax. There is no tax rate you can impose on the larger company to make up for it because the overhead destroys the smaller company regardless of what you do to the larger one.