The Product Of ParallelScale And BBox Display Length Is A Constant

I’m assuming we’re using a parallel projection for 3D environment.
We can find the project transform matrix logic at VTK code source.

//------------------------------------------------------------------------------
// Compute the projection transform matrix. This is used in converting
// between view and world coordinates.
void vtkCamera::ComputeProjectionTransform(double aspect, double nearz, double farz)
<--
  if (this->ParallelProjection)
  {
    // set up a rectangular parallelipiped

    double width = this->ParallelScale * aspect;
    double height = this->ParallelScale;

    double xmin = (this->WindowCenter[0] - 1.0) * width;
    double xmax = (this->WindowCenter[0] + 1.0) * width;
    double ymin = (this->WindowCenter[1] - 1.0) * height;
    double ymax = (this->WindowCenter[1] + 1.0) * height;

    this->ProjectionTransform->Ortho(
      xmin, xmax, ymin, ymax, this->ClippingRange[0], this->ClippingRange[1]);
  }

//------------------------------------------------------------------------------
// The orthographic perspective maps [xmin,xmax], [ymin,ymax], [-znear,-zfar]
// to [-1,+1], [-1,+1], [-1,+1].
// From the OpenGL Programmer's guide, 2nd Ed.
void vtkPerspectiveTransform::Ortho(
  double xmin, double xmax, double ymin, double ymax, double znear, double zfar)
{
  double matrix[4][4];
  vtkMatrix4x4::Identity(*matrix);

  matrix[0][0] = 2 / (xmax - xmin);
  matrix[1][1] = 2 / (ymax - ymin);
  matrix[2][2] = -2 / (zfar - znear);

  matrix[0][3] = -(xmin + xmax) / (xmax - xmin);
  matrix[1][3] = -(ymin + ymax) / (ymax - ymin);
  matrix[2][3] = -(znear + zfar) / (zfar - znear);

  this->Concatenate(*matrix);
}

The ParallelScale is bigger, xmax - xmin and ymax - ymin is bigger, matrix[0][0] and matrix[1][1] is smaller.
So we can get that the display length of model multiply the camera’s ParallelScale is a constant.

The math model about ParallelScale changing is

and its inverse (zoom in and zoom out).
You can see its not linear at drawing tool: https://www.desmos.com/calculator

void vtkInteractorStyleTrackballCamera::OnMouseWheelForward()
{
//...
  double factor = this->MotionFactor * 0.2 * this->MouseWheelMotionFactor;
  this->Dolly(pow(1.1, factor));
//...
}

void vtkInteractorStyleTrackballCamera::Dolly(double factor)
{
//...
  if (camera->GetParallelProjection())
  {
    camera->SetParallelScale(camera->GetParallelScale() / factor);
  }
}