My points 1 & 2 in the previous post are different observations.
Point 1. If I set the width of the track to (say) 3 metres and enlarge the map by zooming, the path that the track is following enlarges 1.5x for each step of zoom on a vector map. while the track enlarges 2x for each step of zoom.
Width = 3 metres: As I zoom in, the track appears too wide. Conversely, as I zoom out, the track gets far too narrow too quickly.
Point 2. If I select (say) chevron markers for the track and set the width of the track to 24 metres, I get exactly the same as if I set the width to 24 px. And when I zoom in or out, the track markers stay exactly the same width on screen. So Locus must be interpreting 24 metres as 24 px in this case. I've not checked whether this applies to all types of track symbol.
Width = 24 metres (or 24 px): Zooming has no effect on the width of the track.
What I want is for the track to stay the same width relative to the path on the map, and for this to work for chevrons as well as for plain lines. I guess that is asking for the width of the line or marker symbols to be defined in the equivalent of "dp" as used in vector maps themes.