Remember the square-cube law; for two things of different sizes, the difference in surface area is proportional to the square and the difference in volume is pr

Remember the square-cube law; for two things of different sizes, the difference in surface area is proportional to the square and the difference in volume is proportional to the cube of the difference in sizes. So, compared to any Earth-bound compost heap, each bit of the sun's surface (wherever you draw the boundary) has more heat-generating interior backing it up.