I'm sorry, I don't understand what "proportion" means in this context. If you are asking how much an exported city needs to be scaled to be the same size (e.g. in metres) in Blender as it is in MFCG (according to the scale bar), then my answer is "I don't know". It depends on how Blender treats coordinate values in OBJ, which are defined in abstract units (as far as I know), not in meters, millimeters, inches or anything like that.