sustainable power is not dependant on muscle size... this is a common misconception. maximal power.. yes and anaerobic capacity somewhat, but sustainable power no.. just look to Rasmussen an other top climbers and top marathon runners as your proof... as you say, they have that "concentration camp look".
for climbing you want the highest power to weight ratio you can muster... to be as lean as possible yes, T-Rex bony girl arms yes ... but you also want your leg muscles to be as small as possible.. just enough to provide enough torque to the pedals to produce the necessary torque. cycling is not a strength sport at all (except for sprinters). to do 500 watts your muscles only need to be strong enough torque the equivalent to squat about 100lbs (50lbs per leg)... so as long as you have the strength of a pre-pubescent girl.. you're pretty much ok. it's not the strength of the muscle that is important in sustainable power it the ability to get oxygen and nutrient in and out of the cells in your leg muscles and utilize it as fast as possible, tiny torques to the pedals over and over again in quick succession and not bit torque outputs... to be a climber you just don't need big muscles. you'll just have to carry them up the hill with you and they don't provide any benefit for the climbing effort. if your an all rounder, larger legs might come in handy in launching attacks, winning sprints etc.
Wiggins has lost 9kg/20lbs over the last two years.. that is a huge swing in his power to weight ratio... look at his climbing in the tour.