The question in the title of your post seems to be slightly different than the question you're asking (above in quotes) in your post. Most serious cyclists I know are told all the time that are too skinny. But what would you expect in a nation where 60% of people are overweight?

I spent the 1st half of July in Italy, in the cycling crazy Veneto region, racing and training, and trust me, I did not feel or look skinny. Most of the people are at an appropriate, "healhty" weight.

My response to the "you're too skinny" comment is to let it go. You can't explain bike riding/training/racing to the average person. They just don't get it, and that's ok.

gene r