On this male Greater Scaup you can see the structural differences in the head and bill that separate it from the Lesser Scaup. The black nail is much larger and the bill actually splays slightly at the end. The muscles at the base of the bill are much heavier, giving the bird a "cheeky" look, evidently they go for larger or harder food items. The big body is wide and quite flat. By weight the Greater Scaup is 20 to 25% larger than Lesser Scaup.
In most lighting conditions Greater show a dark green head color but in certain light maybe even a purple color just as a Lesser usually shows a dark purple head but can show a green head color. As a general rule though, a large Green head in a flock of purple headed scaup usually means a Greater but look at other details for confirmation.