I checked with a guru on this. Here is what he said:

"Here's the deal with line weights in MicroStation. MicroStation defines 

weights in pixels. For that reason they always display the same on the 

screen no matter how zoomed in or out the user is in the view.

Plot widths when plotted or line width values when saved to AutoCAD always 

come from a setting that define a width in millimeters or inches for each 

line weight whether assigned to the element itself or in the level table for 

byLevel weight.

Based on the values listed in the post his dwg files are getting the default 

weight to width mapping in the MicroStation translator settings. He can 

change these by remapping through the csv file which he knows about or he 

can go to the "DWG Save As Options" and change the default values stored in 

the "Line Weights" category of these options. He can access these settings 

by clicking the "Options" button on the "Save As…" dialog of MicroStation 

or by accessing the "DWG Save" options of the Batch Converter."