Although most of the surface has taken on a darker coloration, areas filled with sand managed to preserve its original colour, which is pinkish and thus matches those of other fossils from the San José Formation.

In addition to being incredibly robust, the fossil is also of large size, approximately 500% larger than the known material of the second-largest anhinga species, Macranhinga paranensis.

At the level of the sacral vertebrae there is a notable fossa which is deeper towards its caudal end similar to Anhinga but not as developed as in either Meganhinga or Macranhinga.

The socket that received the femur is closed and notably enlarged and the space between these cavities (of which only one is preserved) is located further ventrally than in the relatives of Giganhinga.

Like the holotype, this fossil exceeds the size of the corresponding bone in any other known anhinga species, which is the main reasoning behind its tentative referral to Giganhinga, as there is no overlap between the material.

Although such a direct approach is not possible in Giganhinga in the absence of limb material, Rinderknecht and Noriega nevertheless calculated the weight based on the proportions of Macranhinga.

[1] In a 2007 paper; Areta, Noriega and Agnolin argue that these results vastly overestimate the weight of Giganhinga, noting that the lack of a similarly sized equivalent makes accurate estimates difficult.

Their results, based on the modern American anhinga rather than extinct taxa, suggests a weight of 12.8 kg (28 lb), only half the mass of previous estimates.

[3] Phylogenetic analysis conducted by Guilherme and colleagues in 2020 recovered that Giganhinga is closely related to the large-bodied species of the genus Macranhinga.

The idea that the size increase is an adaptation to swimming and diving is supported by the anatomy of the antitrocanter, which serves to counteract the forces produced by kicks that birds use while propelling themselves underwater.