I was under the assumption that leg lifting was a dominance thing for dogs. By lifting their leg they can get their scent up on objects, maybe it makes them look bigger the higher the marking goes.

My dog Sadie is a spayed female, yet she lifts her leg. It's not as much a lift as a male dog, she just looks like she's squatting with one leg up a bit. I don't think that indicates dominance in her, she's the biggest wimp, maybe she just thinks she's dominant at the moment.