It is the product of a genetic arms race between mothers and fathers over whether a child should take its nutrition from the mother (by nursing) or the father (by eating). Prader-Willi syndrome occurs when the mother's genetic instructions are not appropriately counterbalanced by the father; the converse -- exactly the same genetic deficiency, but coming from the maternal side rather than the paternal side -- is https://en.wikipedia.org/wiki/Angelman_syndrome .

The same genetic conflict occurs in lions, which is why ligers (lion father, tiger mother) are much larger than tiglons (tiger father, lion mother).

> It sounds like he must have had his metabolism turned up to 11, perhaps due to some genetic mutation that also caused the other abnormalities found in the autopsy?

Tarrare didn't suffer from Prader-Willi syndrome, since as you note he metabolized all the food he ate. I would speculate that his metabolism was sufficient to cause his hunger in the 'normal' way, and the hunger didn't need its own cause.

I would also guess that the enlarged throat and stomach were caused more or less 'mechanically' by his consumption of large quantities of food. A response to his eating habits, rather than a suite of mutations working together to both cause and accommodate a large appetite.

In other words, my causal model would go

metabolism -> hunger -> large meals -> large throat/stomach

In the periods when I've eaten a lot, I could eat a lot. In the periods when I was eating only tiny meals on a regular basis, my stomach would shrink and I would only need tiny amounts of food to fill satiated.