Bob (& others):
In the field, the dusky edges to both mandibles and the olive colored bill of Western Grebe do not stand out against the watery background like the brighter bill of the Clark's Grebe does. I think this gives the illusion of a larger bill on the Clark's Grebe in the field. Also, the angle of the bill, in relation to the observer, can give the illusion of a smaller or larger bill. Perhaps Linda's photos reflect this. I am not aware of any dimorphism/individual differences that would account for the percieved (or actual) difference in bill size.