Bug 34592

If a line with arrow is drawn with linesize 0 (the default), the arrowhead is fairly large, which is ok.
If the line with is set to e.g. 0.1cm prior to drawing it, the arrowhead is rather small and smaller as with linesize 0.
If the linesize of the line with linesize 0 is changed to 0.1cm, the arrowhead
is still larger as of the line drawn with linesize 0.1cm. 
This reverses if linesize eg. 0.3cm is used, but the arrowheads still differ in size.
If drawn with 0.1 and then changed to 0.3, the arrowhead has still another size.

So the size of the arrowhead depends on the history of drawing; 
it is nearly impossible to change a drawing to a new linesize and obtain uniform arrowhead sizes.