I know it is, YJ. But not at the CEO level. Additionally, let's actually look at Barra's CV and Akerson's CV. Akerson may not have had auto industry experience, but he had been a managing director at The Carlyle Group. Bara had been an Executive VP at GM. Managing Director is more akin to CEO than exec VP so one would expect someone who had run an entire company to demand and receive a better compensation package.
Wage disparity between men and women honestly doesn't exist at the top levels of business. A bigger problem there is still the glass ceiling that women have a harder time breaking through. Is that sexism? Probably a part of it is, but another part is, as Krellin points out, a difference in approach to negotiation and pursuit. Men are raised/predisposed to be more aggressive in their pursuit of their dreams. Women are taught/predisposed to be more passive. Breaking the glass ceiling and making things more equal (where realistic, as others have pointed out, women are not as equipped, generally speaking, to handle a jackhammer on a construction site) means breaking the cycle of how we raise our youth. And making those changes won't show results for at least a couple generations. We are starting to do this now with the STEM programs and encouraging young girls to think outside the box, but we won't see these girls turn into women and reach the top echelon of business such that they are no longer the exception, but the norm, for another 40 years.