What is the Document size and the percentage zoom level ?
just normal, has no bearing on document size.
If I just draw a box with the box tool, strokes are normal.
It's only when I place a photo and reduce it in size do the strokes act funny.
It clearly has something to do with reducing the images.
I see there is a preference for scaling objects, and I tried both settings.
But the strokes act funny either way.