Is there some way to disabuse people of the notion that higher taxes would help here? Taxes don't really affect entity size. In significant ways they do the opposite, because business expenses are deductions, so using profits for expansion becomes more favored over paying them out as dividends the more you increase the tax rate.

What you need here is antitrust enforcement and lower barriers to entry so that there are more companies that are each smaller, and correspondingly more millionaires and fewer billionaires.