Athelectasis is probably exclusively a macro "collapse" of some of the airways - my own personal thinking is that bronchopulmonary segments would be spared and if these become involved you are talking more lobar/segmental/total lung collapse - depending.

Either way athelectasis is probably best seen on hemidiaphragms as blurring and less definition of the normally clear edges and its small in size compared to collapse.

But you would find this in basic text books!