I'm guessing most of you guys are just armchair sports fans and never actually spend much time with actual athletes? Maybe this is why the skinny-ness seems to shock you so much. I know lots of serious (and semi serious) cyclists and triathletes and most of them are thin, very thin when compared to your 'average' person in the street (in the UK the 'average' person is about 15KGs overweight). Obviously there are exceptions but most endurance guys who still manage to maintain a heavier/bigger type build tend to be like that genetically i.e. the type of guy who puts of muscle quick with just about any kind of activity but also puts on weight quickly when not training.
Some photos and the angle/light they are taken at make it appear worse. Read Tyler's book - pro cycling has never been 'healthy'. Maybe GH, testosterone etc may decrease muscle loss and help certain riders maintain mass?