I think people get the idea that riders like Lance and Jan are big guys because they see them next to Bettini, Rebellin and Cunego and they look big. It's the rare rider though that pushes 185.
Here's some weights:
Bettini - 128
Boonen - 176
McEwen - 145
A Merckx - 160
Ulrich - 160
Vino - 158
Basso - 154
Julich - 150
Voigt - 170
Cunego - 143
Simoni - 127
Peta - 154
Hincapie - 183
Freire - 139
Dekker - 145
Rebellin - 147