Google has been around for 17 years and employees roughly 10,000+ software developers. I think it's reasonable to assume that the 2B LOC metric is accurate...
Windows has been around for 35 years and Microsoft had 61,000+ employees (ok, that’s not only software developers and they don’t work only on Windows) in 2005; and it’s only ~50M LOC. I don’t think the number of years + developpers really show something; you don’t write new code everyday.
You pointed it out yourself, but I think you underestimated its importance: Microsoft works on many other things. Office, XBox, Windows Phone, Exchange, SQL Server, .Net, etc. I suspect Microsoft's total line count is similar to Google's. The difference, however, is that it's not one codebase.
