As others have said, genes plays a big role.
Also culture too, a lot of the food they eat, even traditionally, was pretty much huge portions of carbohydrates, like sweet potatoes, yams, and plantains.
Lastly, a lot of their meat in the Pacific Islands is really fatty. Good cuts of meat gets exported, bad cuts get imported. The healthier cuts are worth more on the foreign markets like Australia and America, and the locals are relatively poorer so they have to buy the cheap stuff.