If you look at some old pics, it would appear that Fausto Coppi started the trend... dude was as lean as any rider today, complete with fully-visible upper leg veins.
Much like having a high vo2 max, being able to maintain a low level of body fat is one of those things that is individual to a degree, and separates some folks from others. I've seen guys get incredibly lean with no negative consequences, and others just fall apart and get sick every time the wind blows. And, just like oxygen delivery, this is one of those things that gets muddled by doping, and where the order of things can get shuffled.
I have to say though, it's crazy to me to think that Hesjedal was actually 2kg lighter this year, because even in his Healthnet days he seemed unbelievably skinny:
http://autobus.cyclingnews.com/road...p?id=/photos/2007/worlds07/ttpreview/_DSC0042
Pictures don't do it justice, the guy is really skinny. So, if he purposely tried to drop 2kg more, I'm not surprised the wheels came off. Looks like he gambled a bit looking for that last little bit and it didn't work out.