Even though she's a large part of the story, I've always thought of Galinda as supporting. The story isnt about her; its about Elphaba and her transformation.
But Galinda getting the majority of the songs... well, I would posit that this isnt the only case where the lead didnt get the bulk of the score. Look at PASSION. I havent played the album in years, but as memory serves, she doesnt get a whole lot compared to the other characters in the show. (I could be wrong on that point, so feel free to correct; I'm going on distant memory of seeing it on Broadway.) Nevertheless it's pretty clear who's the lead and who's not... and I would suggest that, as much as this is a story about friendship, WICKED is really about the woman who becomes the Wicked Witch of the West and the forces that pushed her there.
YRMV.
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