How to get accurate 3D depth from 2D screen mouse click for large scale object in OpenGL?

I am computing the 3D coordinates from the 2D screen mouse click. Then I draw point at the computed 3D coordinate. Nothing is wrong in the code, nothing is wrong in the method, everything is working fine. But there is one issue which is relevant to depth.

If the object size is around (1000, 1000, 1000), I get the full depth, the exact 3D coordinate of the object's surfel. But when I load the object with size (20000, 20000, 20000), I do not the get the exact (depth) 3D coordinates. I get some offset from the surface. The point draws with a some offset from the surface. So my first question is that why it is happening? and the second question is how can I get the full depth and the accurate 3D coordinate for very large scale objects?

I draw a 3D point with

glDepthFunc(GL_LEQUAL);
glDepthRange(0.0, 0.999999);

and using

glReadPixels(x, y, 1, 1, GL_DEPTH_COMPONENT, GL_FLOAT, &wz);
if(wz > 0.0000001f && wz < 0.999999f)
{
gluUnProject()....saving 3D coordinate
}

I have solved this issue that was happening because of depth range. Because OpenGL is a state machine, so I think I might somewhere changed the depth range which should be from 0.0 to 1.0. I think its always better to put depth range just after clearing the depth buffer, I used to have following settings just after clearing the depth and color buffers.

Solution:

{
    glClearColor(0.0,0.0,0.0,0.0);
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT | GL_STENCIL_BUFFER_BIT);
    glEnable(GL_DEPTH_TEST);
    glDepthFunc(GL_LEQUAL);
    glDepthRange(0.0, 1.0);
    glDepthMask(GL_TRUE);
}