The minkowski() covers tjhe first object with the second object. Whilst I’ve it used to add rounded vertical corners to a cube using a cylinder, using a sphere adds the rounding to all edges!
minkowski() increases the size of the first object, so the cubeRounded() module below first shrinks the cube before applying the rounding resulting in a rounded cube with the height/width/depth as given.
$fn=100;
cubeRounded(50,40,30,2);
translate([60,0,0]) cubeRounded(40,30,20,3);
module cubeRounded(x, y, z, r) {
minkowski()
{
translate([r,r,r]) cube([x-2*r,y-2*r,z-2*r]);
sphere(r=r);
}
}