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Anatomy
Ants, unlike humans, possess exoskeletons, which serve as their only method of maintaining body structure and protection. Their head, thorax, and abdomen are connected and protected by this exoskeleton; and their electrical impulses, when threatened, can send rapid signals through their body to curl in defense, thus protecting their sensitive innards and exposing only their shield-like exoskeleton.
Thief Ants
Thief ants are among the most ubiquitous household pests, and are yellow or golden brown in color. They are most commonly known and distinguished from other breeds by their tendency to curl very tightly into a ball when dead. This is most likely a result of an emergency electric reflex that is sent through the body of the ant, in order to attempt protection.
Red Ants
Red ants are known to "play dead" when threatened, and may curl up into a ball preemptively in order to attempt survival. If killed during this attempted survival, red ants are likely to remain in this position, which may account for their being found curled up after death. However, if they survive, red ants are likely to uncurl after the perceived threat has disappeared and continue home.
Ant Death
It is unclear whether an ant's tendency to "curl up" when dead is related evolutionarily or coincidentally to portability issues. Ants are unique in that they clear their dead (also known as "necrophoric") and dispose of them in an orderly fashion. Once an ant dies, it begins to excrete a hormone called oleic acid, which -- once perceived in high enough levels by other ants -- signals fellow ants to tend to their bodies and dispose of them. It is possible that an ant's instinct to curl up when dead is in some way related to the increased ease with which a round ant body can be carried, as opposed to a fully extended body.