The most common reason for this are layer filters. Layer Filter, though useful, can multiply very quickly. This also causes the Layer Manager to open up slowly.
So to fix this problem you need to delete all the Layer Filters you don't need. This can take awhile if you have a few thousand filters. AutoLISP to the rescue. Our own Robert Bell wrote a routine just for this problem. See the attached file.
Links:
Autodesk Knowledge Base:
ID: TS65282 - Drawing is completely empty but file size is very large
ID: TS21533 - Using the ISAVEPERCENT variable to affect file size, performance, and interoperability
AUGI Forums:
Large file size without any objects in AutoCAD 2002
Huge .dwg files, yet everything is purged
File size!
File Size
increasing file sizes
AUtoCAD loading "junk"
deleting layers
increasing file sizes
Incremental Saves - Isavepercent
how to clear the drawing
Unexplained File Sizes -> Solution found -> Layer Filters
Anonymous Blocks
"Large file size/ Many "Named layer filters"
Remove grouped objects.
Help - Extra Large File Sizes
Huge file sizes
Layer Filters...
File size is gigantic with nothing in drawing.
Drawing file size is big for no reason
file size does not decrease when objects are purged from dwg
WBLOCK = huge file?
Excessively large DWG files
