there is no native import of the STEP file format containing NURBS-based surfaces/solids, during an import by using the SimLab STEP importer plugin the spline based faces need to be triangulated to a facetted mesh which leads to the mentioned big data.
You might want try to export from SU to a 3D mesh format as DAE, 3DS, OBJ and do a simplification of the facetting by remeshing with the free Meshlab.