I find myself having to do this kind of thing fairly often:
ie have a line of smaller text underneath a bigger one, matching widths.
I usually do this by placing the smaller text directly over the larger, then scaling visually (the text bounding box is often bigger than the actual font pixels themselves), then positioning the smaller text. This can be a slow process when you're doing it a lot - is there a quick way to do it? If not in Illustrator, in InDesign?