Not the parent, but I managed and eventually helped lead the paid search group at one of the top search agencies. We had clients with 7 figure budgets all the way up to 9 figure budgets (or essentially unlimited as long as we were within certain parameters).

It's a bit scary and heart attack inducing if something goes majorly wrong (like going dark for a day because someone forgot to check the flight dates in the account after you took it over from the previous agency). But like the OP said, it just becomes numbers.

The cool thing is you not only get much better support and escalation paths at Google, but you can test things at a very large scale and get answers VERY quickly. And little things that might not make a huge difference on smaller accounts, suddenly equate to tens or hundreds of thousands of dollars difference in total revenue. But again, all things are relative, so %'s might not change much.