There are a couple of things you can do. Very likely, the style you chose has Profiles set to 2 or greater. Set them to 1 in the Edge settings under the Edit tab in the Styles window and the thick profile edges will be the same thickness as the rest of the lines.
Another thing you can do is to export the image as a larger size and resize it in an external image editor. In the example, below, you can see how exporting at different sizes and then resizing affects the thickness of the lines. Same line style used throughout. You can adjust the export size by clicking on the Options button in the 2D Export window. This won’t affect the appearance of the model while you’re working but it will show up in the images you make to share.