Justifying last system with multiple pieces

[Francesco Spiga]

Hi everyone. I'm typesetting a choral piece with a 4 parts refrain and a 
monodic verse, and I have this structure:



\book {

%%% REFRAIN

\score { \new StaffGroup
        <<        \new Staff \staffSoprano
                \new Staff \staffAlto
                \new Staff \staffTenor
                \new Staff \staffBass  >>
        
        \layout {  }

  }
}

%%% VERSE

\score { \new Staff  \verse
        
  \layout { }

  }
}
}

When I added the verse \score, Lilypond compressed the refrain last 
system on the left, leaving a lot of blank space on the right.

How to justify it? I didn't find any command to work it out.

Thanks in advance!
F.