By the second metric, Google is not a "large" software system because all code must be reviewed by someone with context on it. (Single-developer projects and codebases don't get anywhere at Google...I tried, multiple times.)

I think a decent definition is "software where no one person has the whole system in her head", i.e. where reading existing code takes as least as much time as writing code, and any change may have unforeseen ramifications on the system as a whole.