Why is cornerRadii parameter of CGSize type in -[UIBezierPath bezierPathWithRoundedRect:byRoundingCorners:cornerRadii:]?

Why is cornerRadii parameter of CGSize type in -[UIBezierPath
bezierPathWithRoundedRect:byRoundingCorners:cornerRadii:]?

I can't figure this out... I'm playing with
-[UIBezierPath bezierPathWithRoundedRect:byRoundingCorners:cornerRadii:]
as such:
bezierPath = [UIBezierPath bezierPathWithRoundedRect:CGRectMake(10, 10,
80, 80)
byRoundingCorners:(UIRectCornerBottomLeft)
cornerRadii:CGSizeMake(20, 20)];
And it works as expected. But if I replace cornerRadii:CGSizeMake(20, 20)
with, say, cornerRadii:CGSizeMake(20, 5) or CGSizeMake(20, 40), there's no
difference.
Why is cornerRadii CGSize and not CGFloat then? What is CGSize.height for?
Any ideas and advice will be greatly appreciated :)