It, may not be from Sumatra but, broadly speaking, it is "Indonesian" (East Timor gained gained its independence in 2002, but is part of the Indonesian archipelago). The preponderance of rats of unusual size in Indonesia can be chalked up to the island effect, which has also resulted in tiny hominids, dwarf elephants, giant monitor lizards, and the like.
Unfortunately, Ursula K. Le Guin didn't incorporate the island effect into her Earthsea series- a Sparrowhawk that preyed on turkey-sized giant sparrows would be quite a beast.
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