HERE IS YOUR ANSWER.

Why are the bottles of medicine and shampoo , etc., not filled up to the brim ?

The reason behind it is very simple. Soft drinks have carbon dioxide mixed up with the liquid part of the drink and is stored in the bottle at high pressure. If there is no space left in between then due to change it temperature and pressure the drink may expand and since there is no room inside the container, force built up leading the container to break/explode. Due to this the precious liquid oozes out and the contents are emptied. Resulting in loss of the buyer. Hence the bottles of medicine and shampoo , etc. not filled up to the brim.