U have two factors, one is genetics. If you have thin legs no matter what u do it will be the same. Second factor is that road cycling doesnt tend to make your legs massive, you will gain muscle but not massive muscle because the excerise is like 70% to 80% aerobic, that means that as long u have good lungs you are all set. Muscles will work by themselves.
Now why trackers have massive legs? (my legs were like the guy ratzinger pictured) simple, track is like 50% aerobic and 50% anaerobic, so u need more muscle in a per points race for example.
A 1 km TT is a race that is like 95% anaerobic and 5% aerobic, it is so violent that u basically have one chance to get air in your lungs and then that's it, like 90% of the race effort is done by bare muscle. Spinters the same thing. Pure anaerobic effort. You just go almost w/o getting any air in. So the body will develop massive legs, thats how it is. The other factor is that track in general is pretty demanding because is not light to ride in the track at all. Roadies when going in a track after a few minutes start having pain in their legs because their legs are not prepared for the load. Many road racers back in the day were doing track in the winter, it helps a lot but if you get too big is bad. EM and other used to do track time to time. Track helps a lot.
Cancelara is a TTist, he will have massive legs because he is supplying lack of oxygen with muscles to get power. So if you are thin you will get muscles up to some point, after that maybe not. And if your lungs are hudge probably the muscles production will be minimum. In my case my lugs capacity never being so big and even now my legs are still thick because i have to supply lack of O2 with more muscles.
Muscles weight more than fat so thats why big guys can't climb very well and trackers basically can't climb.