If you did any effects at all or anything that changes the whole lecture, then Audacity has to save an UNDO of the whole show. If you close Audacity, that should go away because Audacity will not save UNDO. You should go into Audacity Preferences and make sure that it didn’t slide into stereo instead of mono under Devices or one of the other settings under Quality didn’t change.
This can be hidden. If you routinely record in mono (single microphone, right?) but you so much as touch the panning slider to the left of the track, you now have a stereo show whether you wanted one or not, and it’s double the size.
We do note that if you’re worried about 1.3 GB, maybe your next posting is going to be that the computer is stuttering and failing to keep up with the live recording – having run out of resources.