I think I trapped it.
File > Export > (save as file type Block) > (select * for whole drawing)
Resulting file will be smaller - in my case 52Mb to 7Mb.
I have the same situation. Using AutoCAD 2005LT, saving to v2000 yields filesize of 43,869kb. Saving to v2004 yields 7,391. This morning the filesize was 36,000kb, and the only change was in an xref file. It has not grown past 43Mb, so maybe it's topped out. (This is an extremely detailed...