The force of gravity on objects is proportional to the mass of each object.
(That's a big part of the reason why, when you eat more and your mass
increases, you weigh more.)
The forces of gravity between the Earth and the 6kg ball are 50% greater
than the forces of gravity between the Earth and the 4kg ball.
(The gravitational forces between the 4kg ball and the 6kg ball, or between
both bowling balls and you, are so small that they may be ignored.)
