A small phpbb forum i run had a weight of ~344 (steady for a couple months). I recently moved the forum to vB. I imported all of the old phpBB data to the vB forum, and locked the old phpBB forum. Currently i only have the coop setup in the phpBB forum. However, it seems like as soon as i opened the new vB board, my weight was cut in half. Why?
When your indexed page count begins dropping in Google, this is the reason why weight drops. The solution would be to add the co-op code to the rest of your site also.. and begin securing some backlinks for some inner URLs within the old and new forums. This will cause a crawl and your weight should hopefully increase.