too much air indeed makes the tone weaker, even though you dont expect it.
this is becouse you use too much force and becouse of that lose control over pitch and volume.
the key to any breath control is breathing 'trough the stomach' instead of 'trough the longs', if that makes sense written like this; google 'abdominal breathing' if it doesn't.
after abdominal breathing becomes natural, the next step is to sing without 'using' your vocal cords, singing should not use any more force then normal talking, and you should be able to keep it up for almost a full day without any difficulty, if dont right.