At their best, Microsoft's business units are run to think like small entrepreneurial shops but spend big marketing and research-and-development budgets. Employees who feel they have ownership and decision-making capabilities are more likely to do their best than those who feel like cogs in the bureaucracy.
Despite the fact that Microsoft has grown tremendously, most Microsoft teams aren't any larger than they ever were. They just keep getting broken up into smaller and smaller specialties. Instead of a single team focused on the new Excel software, there are a number of "feature teams" that each focus on a certain area of the product. Together they all still strive for the same thing--the best software possible. However, the small teams' size lets their members feel as if they own a problem and can strive to solve it. They can see and feel their impact on the larger project.