Say the square side length is 16. The right picture would have an area of 16pi.
The area of the whole rhombus would be 64.
IF we made a square right around the star, we get a square with 4 quarter circles on each corner.
The square area would be 64 and the 4 quarter circles are just on the whole circle with a radius of 4, with an area of 16pi.
So the area of the cross would be 64-16pi.
Subtracting that from the original rhombus we get 64-(64-16pi) or 16pi.
They are the same.