Strange effect of set-global-staff-size

[David Sumbler]

I am having a weird problem with the title page of a score, and I am
hoping somebody can point me in the direction of understanding what
might be going on.

I am using the same basic layout for score and parts as I have always
used.  But having organized things rather better than in my earlier
projects, I now have a top-level file that enables me easily to select
which scores, parts, midi-files and movements are compiled at each run.
 It all works very well - except:

This evening I compiled the thing for one last inspection before a
final printing.  I then realised that I had not set the staff-size for
the score, and both score and parts were being printed with #(set-
global-staff-size 20).  I use a staff size of 16 for full scores, so I
added the appropriate line at the beginning of the file which prints
the score.  This now begins:

\version "2.19.48"

#(set-global-staff-size 16)

\book {
  \bookOutputName "../AuroraeScore"

  \paper {
    two-sided = ##t
    inner-margin = 15\mm
    outer-margin = 15\mm
    binding-offset = 5\mm
  }

%%% Print score front cover:
  #(define partName " ")
  $(if printCover (ly:parser-include-string "\\include "printCover.ily\""))


It then goes on to print the Table of Contents page and the scores of
the various movements, without any problems.

The file "printCover.ily" is simply this:

\bookpart {
  \markup {
    \column {
      \fill-line { \abs-fontsize #24 \bold { "" \partName } }
      \vspace #14
      \fill-line { \abs-fontsize #48 \bold \titleStr }
      \vspace #1.5
      \fill-line { \abs-fontsize #20 \subtitleStr }
      \vspace #11
      \fill-line { \abs-fontsize #20 \composerStr }
      \vspace #2
      \fill-line { \abs-fontsize #16 \dateStr }
    }
  }
}

The variables are defined elsewhere, and in this case partName = " "

Now for the weird bit.  Without the #(set-global-staff-size 16)
 line at the start of "printScore.ily" my cover page prints just fine,
as it always has.  But with the above line added, the letters are
squashed together horizontally so that they slightly overlap:
vertically everything is fine.

What is even more weird, to me, is that this only happens if one of the
instrumental parts has been printed before the score.  If I comment out
all the requests for parts in my top-level file, the the score title
page prints OK.  This means I can work around the problem by printing
the parts and the score on separate runs.

But can anyone explain what is happening here?  To summarize, the
problem only arises if

(a) one or more parts are printed before the score.  These are printed
with #(set-global-staff-size 20)

and 

(b) I include the line #(set-global-staff-size 16) for printing the
score.

The #(set-global-staff-size 16) works fine for printing the score, so
long as I haven't just printed a part in the same run of Lilypond.

Any help or explanations would be very welcome.

David