I never could understand why gravitational attraction was proportional to distance squared rather than cubed. It's effect goes out in 3 dimensions, not 2.
Think of it this way. If something propagates in just one dimension, then it doesn't spread out at all as it moves along.
Now, think of a ripple in the surface of water. It spreads propagates in 2 dimensions, spreading out as it goes. So, the circumference of the circle is proportional to its radius, and the amount of something following that circle is decreasing linearly on a per unit length of the circumference.
Then think of a ball expanding like a balloon. It is moving in 3 directions, and something on its surface is getting stretched as its surface increases. The surface of a sphere is proportional to the square of its radius. So, the amount of something moving with the ball's surface is decreasing by the square of the radius.
You seemed to be thinking that gravity needs to "fill" the sphere, but gravity is not a material like a gas that stays in the sphere and expands in 3 dimensions like the volume of a balloon. One way that some people think of gravity is to think of "gravitons" carrying the force of gravity between object in the same way that photons carry electromagnetic force from one object to another. In that sense, the number of gravitons per unit area reaching the surface of a sphere will decrease by the square of the radius.
There are many other ways to conceptualize gravity, but they all need to represent the fact that measurements show that gravitational attraction decreases by the square of the distance between the objects because that is what the measurements prove.