I examined influence of body size and mating systems on sexual-size dimorphism by summarizing characteristics and testing for associations among the most dimorphic mammalian taxa—Macropodidae, Primates, Mustelidae, Pinnipedia, Elephantidae, Ruminantia. The most dimorphic taxa were seals in Otariidae. On average, males were three times larger than females, and all otariids displayed extensive dimorphism. Except for the Strepsirhini, most taxa had dimorphism ratios (mass of males:mass of females) between 1.2–1.8. Extent of dimorphism increased with body size but the effect was slight (power function between masses of males and females, 1.04–1.05) for most taxa. Phocid seals and macropodid marsupials had power functions of ca. 1.2. Mating systems were associated with size dimorphism in simian primates and ruminants. Monogamous simian primates were less dimorphic than simians that had polygynous mating systems. Ruminants with tending and harem mating systems were more dimorphic than those with territorial polygynous and monogamous mating systems. Polygyny and how it was conducted were associated with the extent of sexual size dimorphism.