"Lean" has a lot of meanings, but usually, it means "low in fat". The idea is that a company with a lower management ratio (say, 3-5) is flabby. The 1980s change in corporate structure was to "trim the fat". People are still debating whether this was good or bad, and certainly there were companies that had (and still do have) useless people in management, but the long-term verdict has been negative on the new structure. Managing people properly is too time-intensive for one person to have 20 reports.