The problem was:
In qgis the outline was rendered with a 0.26 mm outline.
How much it is in pixels ?
The formulaes are these:
1 meters = 39.37007 inch
1 mm = 39.37007 / 1000 = 0.03937007 inch
0.26 mm = ( 39.37007 / 1000 ) * 0.26mm (the outline) = 0.01023
If the monitor is at 72dpi:
72dpi => 0.01023 x 72 = 0.73 pixel width
If the monitor is at 91dpi:
91dpi => 0.01023 x 91 = 0.93 pixel width
Thats all.
