Jeb Bush has been highlighting the benefits of his new tax plan for the middle class, which in many cases are considerable. But by some measures the biggest winners would be – well, winners.

A new analysis by the business-backed Tax Foundation shows that the biggest percentage increases in after-tax income under Mr. Bush’s tax plan would go to the top 1% of earners, people making more than about $406,000. They would see their after-tax incomes increase on average by 11.6%, according to the analysis. That’s the biggest change...