What exactly happens to a Titan's body after it is killed?
In episode 15, "Special Operations Squad: Prelude to the Counterattack, Part 2" where Zoe Hange has those 2 captured titans (which she named "Sawney" and "Bean"), she mentions that the Titans seem unusually light for how large they are, and that a severed arm of one of the titans weighed nothing, which allows such large bodies to move so quickly with great agility.

Though, at least so far in the anime and manga, there's no explanation to their physiology or what exactly happens after they die, it's probably the case that their solid state is being maintained by their mysterious "power source" and that once the source of power is gone (and thus cannot regenerate), their physical bodies (sans skeleton) return to the form of some type of gas.

So, to summarize this completely unsubstantiated speculation:


Titan's bodies are actually made up of some type of gaseous substance that weighs nothing
While alive, with its "power source", the gaseous substance takes on the form of solid flesh-like material, but still incredibly light
Without this "power source", the flesh returns to the gaseous state and evaporates.
This may or may not have to do with the extremely high body temperature that the titans have.