I agree that core strenght tends to be undervalued, but you will not see a top marathoner with a leg that is quite this size:

http://en.wikipedia.org/wiki/File:Fabian_Cancellara_-_Tour_O...

Form, effecency, and power are important for both sports. However, cyclists focus more on power output.

It should be noted that at 80kg, Fabian Cancellara is considered much too heavy to ever win a grand tour. He is spectacularly fast on the flat because he is only slightly less aerodynamic than a much smaller and less powerful man, but he is quickly overpowered on a mountain stage by lighter riders.

You are correct that different courses favor different body styles, but comparing someone like Paula Radcliffe (http://en.wikipedia.org/wiki/File:Paula_Brooklyn_NYCM_2007_c...) with a top cyclist shows max power is less important in distance running.

Relative to the average American they do have similar thin and muscular body types. And, I think it's easy to underestimate how strong these people are because they have so much less fat than the average person. But, as you say the relative cost of drag for increased body size helps support a more bulky cyclist.

Just FYI Paula Radcliffe weighs 54 kg the cyclist Leonardo Piepoli who is a man and the same height as Radcliffe weighs 52 kg. I do think that cyclists are on average heavier than marathon runners, but the top climber cyclists are not. The advantage Cancellara has on flat terrain is much smaller than the advantage a climber has in the mountains.

In my personal experience with cycling and running I'd say that carrying 1kg of fat running is a smaller disadvantage than carrying 1kg of fat cycling in mountain terrain. This is reversed when you consider cycling on flat terrain.