Because the field scales as 1 / r^2, halving the distance quadruples the gravitational pull. The steep inverse-square climb is what makes low orbits so much faster and harder to hold. The scaling is exact for an ideal point or spherical source; real gravity includes oblateness and third-body tugs, and the radius is a measured quantity with ephemeris uncertainty.
The inverse-square field strengthens at smaller radius. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
The same source at radius one has radius-square one
The same source parameter 16 is now paired with radius 1.
r2=12=1
One over a smaller radius square is stronger
The field rises to 16 because the denominator is 1.