bugs - "Boundary" thickness in HighlightImage

Bug introduced in 11 or earlier and persisting through 11.3

CASE:4076121

---

I'm having a hard time to understand how I can use HighlightImage and only draw the boundary of a binary image with a thickness of exactly one pixel. The documentation suggests that I can use

HighlightImage[
 img,
 {{"Boundary", thickness}, Binarize[img]}
]

but that doesn't work properly. Here is a small example that shows the behavior on Mathematica 11.3 on OS X

img = Import["http://i.stack.imgur.com/DLGJC.png"];
Manipulate[
 With[{img = ImageResize[img, size]},
  Show[HighlightImage[
    img,
    {{"Boundary", thickness}, Binarize[img]}
    ], ImageSize -> 1024
   ]
  ], {size, 100, 500, 1}, {thickness, .1, 10}
 ]

As you will see, the thickness setting has no influence and if you scale the image up with the size slider, then the boundary thickness covers many pixels