Let’s assume you are at 10 meter depth with 8kg weight on your belts. You have X amount of air at your bc. When you rise to 5 meters then your air in bc will expand to 1,5 X. So you have to empty your bc with 0,5 X air to turn back to neutral buoyancy. Yes I know this doesn’t work this much straight, you have to discharge more but this is just to give an idea.
If you are diving with 12kg weight at your belts than X amount of air will not be enough for the same diver at 10 meters, he must have 1,5 X air (again assumption not a certain number, just to give an idea) When he ascents to 5 meter than air in his BC will be 1,5 X x 1,5 = 2,25 X. This means he has to discharge 1,25 X amount of air.
Discharging 0,5 X amount of air can be controlled much more easily in a shorter time than 1,25 X amount of air. It is much safer. Also using your air in bc is a bad way of wasting precious air. This is not the only way to loose air. As you need more adjustment at your bc when you are changing your depth you need to use fins to stay at the same depth. This makes you use energy which is equal to using air.
Learning to use lungs for buoyancy doesn’t mean to have excess air in your lungs for a long period. This is just for small adjustments. Then you have to make the real adjustment with your bc. Give a normal breath, than try to give more breath. You can easily see there is still air in your lungs after you give breath normally. Try to stay neutral after you give your breath normally. If you are neutral, breathing normally will not change your depth in a sudden.
When I dive with a group with new divers that’s the only case I dive with excessive weight because I might need to bring them down when they start to rise uncontrolled. Otherwise diving with right weight saves air and it is much safer.