where are diminished and half-diminished symbols defined?

[Mark Polesky]

Where in the source are the chord symbols for diminished
(like °) and half-diminished (like ø) defined?  Are they
glyphs, characters, markups or what?  I looked but I can't
find them.  And why is the half-diminished sign so much
bigger than the diminished?  Is that intentional?

\chords {
  c:dim
  c:min7.5-
}

Thanks.
- Mark