Yes, the covered surface is much larger. Frankly I don't get why MS insists on keeping Windows closed source. I get why you'd want to keep something others might be able to easily reuse closed source, but I don't get why Windows developers don't get to see Windows source code. In fact, having worked with Linux extensively for the past decade, I don't think I'd be comfortable doing serious work on a system where I don't have source code for debugging and documentation.