It has been estimated that "P. brasiliensis" reached about in length, weighing about . Another estimate gave a larger size of in length and in weight, with a mean daily food intake of . However, these proportions have been met with a lot of scrutiny, and it is more likely that "Purussaurus" reached only long and . As only skulls have been found, the actual length is not certain. Bite force has been estimated to be around 52,500 N (around 5.3 metric tons-force). The large size and estimated strength of this animal appears to have allowed it to include a wide range of prey in its diet, making it an apex predator in its ecosystem. As an adult, it would have preyed upon large to very large vertebrates such as the xenarthrans and notoungulates present, with no real competition from sympatric, smaller, carnivores. Researchers have proposed that the large size of "Purussaurus", though offering many advantages, may also have led to its vulnerability. The constantly changing environment on a large geological scale may have reduced its long-term survival, favoring smaller species more resilient to ecological shifts. In other words, it was over-specialised and couldn't survive when its habitat changed, unlike smaller related species of caiman.