The fact that the total share of MS has been shrinking doesn't necessarily mean that MS is a small player in the global market.
That depends on the context. If you are trying to decide which OS to develop your software for, market share (with some adjustment for the willingness of users of a certain OS to actually pay money for software) is the critical factor. While Microsoft may still be large in terms of number of installations or dollars, losing on market share is significant. In the past, most software was developed for Windows first, and then OSX or Linux if the developer got around to it, which gave Windows a huge advantage in the market regardless of whether or not it was the best from a technological standpoint. Also, market share allowed Microsoft to essentially dictate "standards" (remember all of those websites that required ActiveX?). Microsoft may still be big and wealthy, but losing on market share really does change things -- it makes them "small" in a sense.