Why vtkMetaImageReader read a .mha file, the memory blast?

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vtkSmartPointer imagereader = vtkSmartPointer::New();
imagereader->SetFileName(filename);
imagereader->Update();

this is the code when i read a .mha file just 2.3M, the program memory expand to 1 G?
this “filename” just a .mha file file path.