Thank you for your answer Jean. Infact i already work very organised. All the models i insert are already components. So copying instances is not a problem. Also no loose geometry or ungrouped objects.
As i searched deeply, i found out that the main problem occurs when i export/import the models.
The 550kb fireplace which i designed in 3ds max becomes 5mb when i export as fbx. And when i import it in sketchup via simlab's fbx importer it becomes 75mb...
I have reported this to both autodesk and simlab. Waiting for an answer if there is an other way of exporting/importing without growing the file like this.