I also noticed that the obj probably contains a lot more than just the used objects, it looks like it exports all the installed libraries because i see objects in the UE project that i actaully didnt even use in my sweethome3d project.

How did you export the project?
If you only want what is visible on the levels you have, select "Select all on all levels" from the edit menu and then export to OBJ. That result should not include anything that is not a part of the design.