Male southern elephant seals are the largest pinnipeds, larger even than northern elephant seals, Mirounga angustirostris, their closest relatives. Mirounga leonina males have been documented reaching over six meters long and weighing over 4000 kg. This is in sharp contrast to females, which are rarely over 800 kg or four meters long. In fact, both species of the genus Mirounga are more sexually dimorphic than any other mammal. This dimorphism stretches beyond just size. Males also have a large, inflatable proboscis, which enhances vocalizations used to challenge other males for mating rights. The southern elephant seal proboscis is slightly smaller than the proboscis of northern elephant seals, overhanging the mouth by only about 10 centimeters compared to 30 centimeters in their northern relatives.

Breeding populations vary in size. In the South Georgian population, males average 450 cm in length and weigh 4,000 kg. Females average 280 cm and weigh 900 kg. The seals from Macquarie Island population are somewhat smaller, with males averaging 420 cm in length and 3,000 kg and females averaging 260 cm and 400 kg. ("Elephant Seal", 2002; "Elephant Seals", 1983; "Elephant Seals", 2002; "Southern Elephant Seal", 2003; Anderson, 2003; Briggs and Morejohn, 1976; Crown, 1997; ESRG - Filippo Galimberti and Simona Sanvito, 2002; Gaskin, 1972; Nowak, 2003; Seal Conservation Society, 2001; Slip and Clippingdale, 2002; Van Der Toorn, 1999)

Despite the large difference in size, male and female southern elephant seals do share many physical traits. They have a similar body type. This includes short front flippers used primarily for steering in the water, and very strong, fully webbed, rear flippers that can propel them through the water with remarkable speed and agility. They also have a layer of short, stiff hair covering their bodies. At birth this fur is very dark in color, but lightens after the first molt. New fur after a molt is typically a dark gray/brown with lighter underside and lightens over the course of the year. It is also common for the bodies of both sexes to have scars, usually around the neck, from fighting and mating. ("Elephant Seals", 1983; Anderson, 2003; Briggs and Morejohn, 1976; Crown, 1997; Gaskin, 1972; Nowak, 2003; Seal Conservation Society, 2001; Van Der Toorn, 1999)