Multiplot: Size of plots varies?

Hello everyone..
I am trying to get a multiplot consisting of 1 row with 3 columns. I set the size of all three plots in the same manner but the first plot in the row is smaller than the other two? I can't spot the mistake. Can anyone help?
I am using gnuplot 4.2 Patchlevel 2. Here is my script:

reset
set term pdf enhanced size 13cm,5cm
set output '3p_fields_e2min.pdf'
set title "{/Symbol h}_2=(1-{/Symbol r}_0)/2"
set xlabel "Tuning [deg]"
set ylabel "Power [a.u.]"
set xrange [-90:90]
set multiplot layout 1,3
set origin 0,0
set size 0.33,1
plot '3p_fields_e2min.dat' usi ($1):($3) ti "|b_2'|^2" w l
set origin 0.33,0
set size 0.33,1
plot '3p_fields_e2min.dat' usi ($1):($4) ti "|a_1'|^2" w l
set origin 0.66,0
set size 0.33,1
plot '3p_fields_e2min.dat' usi ($1):($2) ti "|b_1'|^2" w l
unset multiplot
set term aqua