This is quite an interesting one. Try as I may, I was unable to use any of my preferred methods to decorrupt this one. We had an AutoCAD drawing that was about 20 MB in size, only 2 MB with just 1 line in it. Saving in the current format (2013) dropped the file size to about 10 MB, then purging brought it down to about 5-7 MB. Starting a new blank drawing and copying 1 line made the new blank drawing with 1 line about 2MB in size! Looking at the linetype table there are 100's of strange looking linetype names.
Typically these linetypes are inserted via Microstation then exported to AutoCAD, or are within a block that came from a Microstation DGN file and once it's in there it attaches to seemingly everything.
So, how can you tell if this is the problem you are experiencing? Here are a couple quick tests for you.
Two tests for you to verify this is the right solution for you:
1) If your drawing has a lot of line types and you start a brand new drawing and import say 1 line and it brings a lot of stuff in with it (100's of linetypes) and the drawing is way too big for just 1 line, then try test #2.
2) Save the drawing to a DXF, open the DXF in Notepad and search for DGNLSDEF. If that exists then you may want to get this new utility Autodesk has posted for download.
Autodesk has a blog about this particular issue and some code to strip out the unwanted linetypes (100's in the drawing I had).
Just the other day Autodesk posted a utility to fix your drawings too! As I was about to post this blog, they removed it, so keep your eyes open for it here. http://usa.autodesk.com/getdoc/id=DL21934581
* * * Update * * *
Autodesk has posted a hotfix for this now.
Note: the hotfix is only for the 2013 & 2014 programs
or you can use the code from their blog: