Problems with literal fonts

[Paolo Bacchetta]

Fonts in title and authors are simply way too big!! Even with no notes 
Title, Authors etc are rendered so big that it is impossible to read music

\version "2.18.2"

\header {
  title = "Prova"
  composer = "P. Bacchetta"
}

global = {
  \key c \major
  \time 4/4
}

jazzGuitar = \relative c' {
  \global
  % Music follows here.
  
}

\score {
  \new Staff \with {
    midiInstrument = "electric guitar (jazz)"
    instrumentName = "Jazz guitar"
  } { \clef "treble_8" \jazzGuitar }
  \layout { }
  \midi {
    \tempo 4=100
  }
}