Hans, Mike is clearly speaking about the default size displayed in the import model wizard.
This default size is read from the 3D model file and as the OBJ file doesn't specify the unit of the decimal values it contains, the wizard can't guess if it should multiply or divide all these values by a given factor to get centimeters.
Anyway from the many 3D models I have been handling during the last years, I can tell you that very often, the size of 3D models doesn't match real size.