The wing taper is one thing also, but it really appears that if you overlaid the planform view, in addition to the wing taper, you would see a longer tail moment, and a larger tail, even more so than the difference in wing area. I think just those two things alone might make a bigger difference than the swept LE. I guess sweeping the LE also allows the flap taper to be more gradual, and still giving the desired root:tip ratio.
R,
Chris
PS. Hope someone will let me know if these observations are correct. Tough to say from just looking at a picture.