True, the OpenGL camera model isn’t terribly intuitive 
You’ll want to transform the model matrix of the camera. My python is a bit rusty, but something like
renderer = ...
cam = renderer.GetActiveCamera()
transform = vtk.vtkTransform()
transform.Scale(s_x, s_y, s_z)
cam.SetModelTransformMatrix(transform.GetMatrix())
should do it (note that it’s the ModelTransform, not the ModelViewTransform).