I think faulty_damper is right to a large extent. In pretty much all societies, women are generally discouraged to express their own thoughts and desires, and men are encouraged to do so. This includes display of emotions. Some societies are very strict about this, others are more relaxed, but the general trend is obvious. Of course, this is terrible, but I think it does account for the observation. The same is true in all kinds of other activities, such as cooking (how many great female Chefs are there, given the fact that there are much more females who cook?), chess (there is only one female player in the top crop), business, etc.
Looking at professional pianists is not a good way to assess the situation, because those can, by definition, play expressively, otherwise they wouldn't be "good" enough to be professionals. Piano students, I believe, are the right group to examine, and here, I see the same correlation to society as I have mentioned above.
IMO, physical differences are negligible as can be seen by so many outstanding female pianists. By the way, I agree that Olga Kern is one of the most captivating pianists. I was holding my breath too, just like Motrax, until I was blue in my face. What is unfortunate, however, is that I have heard many people (particularly men) complaining about her display of emotions (just check out her reviews on Amazon). I haven't heard anyone complain about any male pianists in this respect.
Women can, intrinsically, play expressively, just as they can, intrinsically, play chess and run companies, but they are not necessarily encouraged in their efforts.