I think it’s referring to the fact that the big asteroid could fit four medium asteroids in terms of area, while only two medium asteroids come out when it gets

I think it’s referring to the fact that the big asteroid could fit four medium asteroids in terms of area, while only two medium asteroids come out when it gets destroyed. This is the same for medium to small asteroids. In the comparison image from one large to four small asteroid you can see the reduction in total area of the asteroids.