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 HVAC drawing from an outside engineer. We never reach this level of detail on our own.)
We have been battling (deleting) layer filters for months. Service Pack "AutoCAD LT 2005 - English SP 1" seems to clean out bogus layer filters (those referencing non-existent layers) merely by opening the Layers dialog box, which also opens much more quickly than without the service pack applied.