Change size of notes in addQuote passage

[Owain Sutton]

I want a full quote of the line, including slurs/dynamics/etc. so it's playable 
by a different instrument if the intended one is not present, so I'm not using 
Cue instead.  However, I want the quote to be with reduced size noteheads. This 
override doesn't cause any change:

\addQuote sopQuote {
  \transpose c' c {
      \override NoteHead #'font-size = #-3 \sopCornet
        }
    }

What's the best way to go about it?