The reason that top pros today are thin is simply because they train a lot. They carry less weight than in previous times, the diet is very carefully controlled and monitored. It's has zero to do with illegal chemicals or methods - just have a look at other sports and see what the effect of their training is.
In answer to an earlier question, a stage like today's will eat up about 7,500 - 8,000 kcals. It's hard to eat that much, which is where gels and the like come in, plus a lot of rice (favoured over pasta nowadays).
Many years ago, when even I was a lad, one of the French magazines published a whole page of rider portraits shot at the beginning of the TdF, at the end of weeks 1 and 2, and after the finish, titled "Le Tour, il change leur visages". It sure did. More gaunt as the race went on, because the modern diet was not around, and people eat steaks fior breakfast (which didn't digest properly) and all kinds of stuff, but simply could not get the correct kcals in, or often get decent quality food at all.
Everyone still loses weight on the TdF (andother stage races), but not quite so much and without, generally, too much loss of power.
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