The US wouldn't need a legal limit on wealth to prevent Google from becoming this large.
Every major Google product since gmail in ~2004 was acquired. Google Maps? Acquired from Where 2 Technologies. Google Docs? Acquired from Writely. Android? Acquired as a startup. Google Analytics? You guessed it, acquisition. DoubleClick? Once again, acquisition. Deepmind? Acquisition. reCAPTCHA? Acquisition. Youtube? Believe it or not, acquisition.
This isn't a story of a business getting big because of their innovation and the vast demand for their much-loved products.
This is a story of US competition regulators sleeping on the job for 20 years.