Bottles with liquids need to have an air space at the top of the bottle. This allows for the liquid to expand and contract when the temperature changes, and helps to prevent leakage. The different size bottles are each made a little bit large for this reason. This way the full amount of oil can be added to the bottle without the bottle being overfilled. A one oz bottle that has one full oz of oil will not be filled to the very top. Nor will any of the other bottle sizes.
The photo below shows each size bottle when it is perfectly filled to hold exactly the amount of liquid that it is labeled for. Someone may think that a bottle is not completely filled, but if these were filled any more, they would have too much liquid in them.