Altering font size using label expression?

Yes! One can definitely set different font sizes for different fields within a single label. Please see this Link: http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#//00s800000027000000.htm

Look at the font attribute. You can set a particular size for the important part of the label and a smaller size for the less important part.

I should be reading the help more often...