I know some tall girls (5'9+) who date guys who are shorter than they are. Most girls are shorter than the average guy, so finding a guy smaller than them really narrows the field to a few guys. That's the main reason you see mostly taller men with shorter women in a couple -- statistically, it's the norm.
Whereas, most women SAY they prefer a guy who is at least a bit taller than them, most would still go for a guy they have great chemistry with if he's around their height. I'm 5'6 and have dated guys who are also 5'6, but barely even know guys who are under 5'6, for instance.
Also, some women are self-conscious about their own size and have been told (much as men are told about height) that men seek to be larger and more powerful and are most attracted to women who make them feel that way. Some may turn away from smaller men for fear that the man would resent this.