Sometimes when opening a DWG file and placing lines and/or smartlines they place very large width. This can occur after opening the file in MicroStation after it has been last edited in AutoCAD.
The polyline width value has been set to an extremely high value in AutoCAD or even possibly in MicroStation. Any subsequent lines that are placed in the DWG file when open for editing in MicroStation will result in very thick lines. This will occur with any linestyle or weight value that is actively set in the file.
Use keyin "dwg plinewid <0>" i.e. dwg plinewid 0
Plinewid is a sytem variable in the DWG file that stores the default polyline width. If this value is set to a high number all lines and smartlines will be placed at the assigned polyline width valure regardless of lineweight settings in the file.
See the following video:
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