Sometimes line weights when displaying a model are heavy / thick and not what the default should be in MX. see picture below.
This is usually due to the incorrect View Attributes being set on the display. You can access the View Attributes dialogue by clicking on the View Attributes button in the toolbar at the top left hand corner of the display. It will open up the panel shown below :
Ensure that Line Weights is not checked / highlighted, and the lines should now appear with a normal weight, as shown below :