My weight went from 4600 yesterday to 839 today, looking at the back link tracker tool nothing has changed, still the same number of pages indexed and same PR. The weight has been steady for a few weeks now so a drop of this magnitude comes as a complete surprise. It seems to make an instant difference in traffic numbers, today's traffic is down by 1/2 . See my avatar .
The KWT doesn't update the metrics daily, only when a new PR is found. Delete the site from the backlink tracker and add it again and you will find most pages disappeared probably.
So the pages we see with the tool may in fact not be there, so thats why the weight is down. That would explain why the weight fluctuates but everything still looks the same. The frustration continues when I do a Site: search and find more pages have been added. It's like a yo yo where the tools can't keep up I guess I should only have a look once a month, in fact I don't know why I bother, it's pretty much out of my control anyway. So long as there is a steady climb everything is ok.
You need to check in the API for when pages shift. Not google.com. Also, I saw my weight do the same thing yesterday 5500 -> 800 or so, but it came back. Check the API a few times, see if it's just fluctuation, you might recalc your weight and see if it comes back. (What's really disappointing is when you see your links in the API jump from 5k to 10k, but only stay that way for about 3 queries, then back to 5k, for no apparent reason. =])
Well... you could be worse... I have site that was showing over 25k indexed pages and gave me 19k in weight. Then stupid google decided to wipe out over 10k pages and I am left with a lousy weight of 1,500