If two similar figures have side lengths that differ by a factor of x, then their area will be different by a factor x².
The side length of Hunter's square of compost is 4 times larger than the 6 by 6 square. The area will be 4², or 16, times larger than the area of the 6 by 6 square.
Multiply 16 with 500 beetles.
Hunter can have 8000 beetles with his square of compost.