so the ‘thickness’ … or boldness etc of a line is (i believe) more of a pixel dimension… its size appearance has nothing to do with what units you’re modeling in, or how long the curve is…
it doesn’t work something like:
default curve width is 1" wide… and a bold curve is say 2" wide.
as a contrasting example, dimension sizes are set to a particular size in relation to your model… if my dimension’s text is set to 3" tall and i zoom out, the dimension will appear to shrink on the display…
but with lines, they don’t have any true thickness… a line is 0" wide and we can’t actually see them… the lines we see on the display are representations of an invisible object.
so, (in rhino at least), the line will always be 0" wide regardless of how many pixels we’re telling rhino to represent them to us at… likewise, the pixel dimension will stay the same regardless of zoom level.
to do what i think you want, you’d probably have to set your viewport to the size/zoom level/etc prior to doing the lineweight adjustments… then save this view… and don’t mess with the zoom levels… or if you do zoom in to do some details, do so knowing that the final image will be at the level of your original scene… restore view if need be.
otherwise, yeah… draw the rectangles on top of the lines.
except realize when doing this, you’re out of the realm of what a line is or what a bold line is.
or- you’re no longer asking (at certain levels), how do i make a bold line? because it’s not a line anymore.