Center two fonts with different different sizes vertically in an NSAttributedString

I’d say the easiest thing to do is just manipulate the NSBaselineOffsetAttributeName attribute for the text in question:

NSBaselineOffsetAttributeName

The value of this attribute is an NSNumber object containing a floating point value indicating the character’s offset from the baseline, in points. The default value is 0.

To center, you’d take the difference between height of the large text and the height of the smaller text and halve it, then use that as the baseline adjustment.