IThe reason you see somewhat overweight riders in moderate to fast group rides is that americans tend to be overweight anyway, and unfortunately cycling at a moderate to moderately hard level you can still be overweight, unless you control what you eat. (i know this all to well). Because it's not weight bearing, cycling just doesn't place enough penalty on carrying extra weight to select out heavy people, until you get to a racing level of intensity, or lots of steep climbing.

contrast running where you do bear your weight, and very few runners that could be categorized as in the fast group are carrying many extra pounds.