President Trump and Ursula von der Leyen, the president of the European Commission, clearly agreed on one thing as they announced the outlines of a trade deal between their two massive economies. It would be huge.

Ms. von der Leyen had emphasized the potential scale during negotiations, and she reiterated it after the two sides announced an agreement Sunday, calling it “the biggest trade deal ever.”

Mr. Trump picked up that talking point and ran with it. “This is the biggest of them all,” he said.

For a president who often fixates on superlatives, the new pact offered an attractive talking point. The United States and the 27-nation bloc have the largest economic relationship in the world by many measures, trading nearly $2 trillion in goods and services per year. But while that may have helped Ms. von der Leyen to get an agreement over the finish line — despite Mr. Trump’s longstanding skepticism toward Europe — a giant deal is not necessarily a good deal for Europe.