Scaling a TIFF Volume

Hi community,
when reading a volume as a TIFF stack using the vtkTIFFReader the size of the volume can not be scaled using the readers SetDataSpacing function.The SetDataSpacing(x,y,z) function don’t seem to do anything.

how should i modify it.

thanks!

vtkSmartPointer<vtkStringArray> fileArray = vtkSmartPointer<vtkStringArray>::New();
    for (const auto& file : filesName)
    {
        vtkStdString::StdString fileStr(file);
        fileArray->InsertNextValue(fileStr);
    }

     vtkSmartPointer<vtkTIFFReader> reader = vtkSmartPointer<vtkTIFFReader>::New();
    reader->SetFileNames(fileArray);

    reader->Update();

    int dimensions[3] = { 0,0,0 };
    reader->GetOutput()->GetDimensions(dimensions);

    reader->SetFileDimensionality(3);
    reader->SetDataExtent(0, dimensions[0] - 1, 0, dimensions[1] - 1, 0, imgUsedTotal - 1);

    reader->SetDataSpacing(1.0, 1.0, 3.0);
    reader->SetDataOrigin(0, 0, 0);
    reader->SetDataByteOrderToLittleEndian();

Can people give me some advise? thanks!