One repeats the following process with lead pencils.
A. Sharpen to a cone pointed tip.
B. Flatten the head through use.
So if you divide the "cylindrical" lead structure in the pencil along the length into mini cylinders. Then you basically remove 67% the volume by sharpening the mini cylinder and using the conic remainder.
So you lose at least 67% of the lead in the pencil by sharpening. You can save unnecessary waste of wood and lead by not using lead pencils.
TIP: A cylinder has 3 times the volume of a cone of the same base and height.