having the same **** problem here, still not yet found any proper solution, but did you try import back that fbx file into your Blender or 3ds and compare with the object you made? somehow when i import the fbx file back to the scene i created in my cinema 4d, the object become very small compare to the model before i do export, some say it’s something to do with the exporting, some say your project scale need to set to centimeter since Unreal use centimeter as their unit, but the problem still there.