I think that the XP gained through skirmish games should be reduced and the XP gained through multiplayer games be increased.
The AI is dumb(sorry to say this) so playing against them shouldn't give that much XP. Meanwhile, people are more intelligent than AI and they can also change their tactics or strategies upon facing a different situation, so I think that playing against players should give more XP points.
However, people might find new ways to play against themselves which could give them more XP points, I mean two accounts of the same person. Which I don't know how would this be fixed or resolved.