A heavy crest can be a sign of insulin resistance, whether the horse is otherwise fat or not. We have our insulin resistant 8 year old show mare on a low carbohydrate diet plus Remission, a magnesium/chromium supplement for horses with heavy crests. The real danger with the heavy crest is not how it looks when it falls, but that it may indicate a high risk for founder. That is why the Remission and the low carb diet. I feed low NSC grass hay and Purina Well Solve L/S grain. No sweet feed! Trust me, you want to do everything you can to avoid founder!!!
You can also reduce the crest some using neck sweats, but you need to do it before it falls. Lots more info if you Google "Safergrass".
Somewhere I found a formula to determine if the crest is heavy or not - the ratio of the neck diameter half way between poll and withers to the neck length from poll to withers should be below some number.. I will try to find it, or maybe someone will post it.