Converting STL to STP for CAD/CAM.
I am converting an STL file into step using FreeCAD, during the conversion the file goes from MB to GB. I have checked the procedure, others also use but what's the way out. Everyone knows the pain of handling huge files, it will take a day or two for importing it.
I often do the following to get the best results to reduce the STEP size without trading much in quality.
1. Reduce the polygons in your STL without losing much of the quality.
a. Install MeshLab.
b. Import your STL
c. Filters -> Remeshing, Simplification and Reconstruction -> Quadric Edge Collapse Decimation
In this dialog, choose reduce the Target number of faces with a Quality threshold of 0.1 to 0.3.
After applying you can see the parameters change at the bottom with remeshed number of vertices.
d. Save/Export your STL.
2. Import STL in FreeCAD. Go to Edit -> Preferences -> Import-Export -> STEP
Here you can try to toggle the "Write out curves in parametric space of surface" to see the file size difference.
Another alternative is to import the STL file in a solid modelling program and reconstructing them and deleting the original body.
Also, you can reduce the polygons using MeshMixer or Blender too. But you cannot reduce the STEP size of an STL that easily because it contains too many polygons most of time. The polygon count is a quality parameter in STL format and file size factor in STEP format. A battle in conversion is a loss to one. :)