What you show is consistent with what I see. The line weights in the .svg exported from Composer are the same whether zoomed in or zoomed out. If zoomed out where the image is small then the lines will appear thicker and that really makes sense. It means that you need to make the right aesthetic decision when you are setting how far you want to be zoomed in on the image when you save an svg out of Composer.
My problem is that I sometimes need to change the scaling of the imported Image in Corel depending upon where I am placing the image on the page, and then things changed. It does occur to me just now that maybe I shouldn't be doing that Scale with Object step to all the lines in Corel.
Also, a different issue but related: When I bring in an svg. It always puts the text "tooltip" in the upper left hand corner and I have to ungroup(explode) the .svg and remove it along with a couple no fill boxes. What's that all about?