The tadpole of Pseudis paradoxa grows to about 22 to 25 centimetres including the tail, which makes it one of the largest frog larvae known. The frog that emerges from it is around six to seven centimetres in body length. Early naturalists who encountered both stages assumed a mistake or a transformation from frog into fish, which is how the species acquired the epithet paradoxa.
The shrinkage is real but partly an artefact of how tadpoles are measured. Much of the larval length is tail, which is resorbed at metamorphosis, and larval tissue is watery and low in density. Even allowing for that, the tadpole's body alone is large relative to the adult, and mass falls during the transformation.
Why the larva grows so big is unresolved. A large tadpole is beyond the gape of many aquatic predators and can store reserves for a rapid metamorphosis, but the trade-off is a long larval period in permanent water. Adults are highly aquatic, using large webbed hind feet, and rarely leave the pond.