I use NSAttributedString to generate a string with two different sizes. By default, its bottom alignment looks like this:
I'd say the easiest thing to do is just manipulate the NSBaselineOffsetAttributeName attribute for the text in question:
NSBaselineOffsetAttributeNameThe 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.