Understand that Google is continuously rewriting their infrastructure to handle increased scale. That's what they have 25k employees for.
Jeff Dean's rule of thumb is that you should build a growth factor of 10 into the design, but any more than that and you will probably have to re-architect anyway. So going from 10,000 to 1 million and 1 million to 100 million are probably roughly equivalent in difficulty.
A fairly large percentage of them, and many of the ones whose direct job responsibility isn't infrastructure (like me) frequently have to deal with the consequences of building for scale as they develop features.
Jeff Dean's rule of thumb is that you should build a growth factor of 10 into the design, but any more than that and you will probably have to re-architect anyway. So going from 10,000 to 1 million and 1 million to 100 million are probably roughly equivalent in difficulty.