What is the primary visual clue regarding size when identifying an eider duck in the water?

Answer

They are significantly larger and more substantial than a Mallard.

Eiders are among the largest sea ducks likely encountered in the Northern Hemisphere. Their heavy body mass gives them a powerful appearance and makes them noticeably larger than ducks like the Mallard when compared side-by-side.

What is the primary visual clue regarding size when identifying an eider duck in the water?
birdidentificationduckEider