Gyrfalcons are markedly bulkier than peregrines, with females weighing up to about two kilograms, and their wings are broader and less sharply tapered. That build favours power and endurance in level flight over the extreme terminal velocity of a stoop.
The prey drives the design. Across much of the Arctic gyrfalcons depend heavily on ptarmigan, ground-dwelling grouse that flush low and fast over open tundra where there is little cover and few high perches to dive from. A long, low tail chase, often ending with the falcon forcing the prey to the ground, works better there than height.
Colour is the other point often misstated. White, grey and dark morphs are not separate species or subspecies in current treatment, and the white form predominates in Greenland while darker birds are usual elsewhere. Morph frequency varies geographically, but the boundaries between forms are gradual.