global-set-staff-size not equivalent to layout-set-staff-size?

[Patrick Karl]

The following snippet is engraved differently depending on whether the 

        #(set-global-staff-size 9) 
           or
        #(layout-set-staff-size 9)

is in effect.  The difference is that the "set-global-staff-size" command 
causes the piece to be engraved on a single page, while the "layout-set-staff 
size" command causes the piece to be engraved on two pages.  The reason appears 
to be that the layout-set-staff-size command doesn't appear to actually change 
the staff sizes.

It was my understanding from reading the Notation Manual that the only 
difference between these two commands was scope, the "global" version affecting 
the entire document while the "layout" version only affects the score it is 
contained in.

(This problem arose when I wanted to combine several "pieces" into a single 
file but retain the earlier pagination, which depended upon different values of 
the "set-global-staff-size" parameter value.  I have not yet been able to 
discover how to do the combination and retain the original pagination due to 
this difference in behavior.)

Can anyone clarify?  Thanks.

\version "2.12.3"
%#(set-global-staff-size 9)

S  =   \relative g'{
 \repeat unfold 10 {
        a4. gis8 a4 a8 gis a b
        c1~ c4
        e4. dis8 e4 a8 gis a b
        gis1~ gis4
        a4. gis8 a4 c8 b c a \break
  }
}

A =   \relative b' {
  \repeat unfold 10 {
        R1*5/4
        a4. gis8 a4 a8 gis a b
        c1 r4
        e4. dis8 e4 fis8 gis a b
        c4. b8 c4 e8 d e c \break
  }
}

T = \relative b' {
  \repeat unfold 10 {
        a4 c e d b
        a4 c e d b
        c4 e g f d
        e4 gis b a gis!
        a4 e c a c \break
  }
}

global= { \tempo Allegro \time 5/4 }
\score {
        \new ChoirStaff <<
                \new Staff { \global \S  }
                \new Staff { \global \A  }
                \new Staff { \global \T  }
        >>
        \layout {
                #(layout-set-staff-size 9)
        } 
}