Why the exported step files are so large?

Such as title. My code is :

STEPCAFControl_Writer Writer;
Writer.SetColorMode(true);
Writer.SetDimTolMode(true);
Writer.SetLayerMode(true);
Writer.SetMaterialMode(true);
Writer.SetNameMode(true);
Writer.SetPropsMode(true);
Writer.SetSHUOMode(true);
Writer.Transfer(Doc, STEPControl_AsIs, "");
Writer.Write(Path);

The step files exported by UG is 450kb, but exported by the code above is 4.30MB.