Every product we fill has two checks to ensure that the correct amount of product is in the tube. The filling machine measures out the correct amount each time. So if a tube has a 15ml fill the machine will dispense 15ml of gel or cream. The actual size of the tube is not relevant to the machine. So it will not fill to the top. It will fill the amount that is stated on the tube. So you might find air at the top of the tube or some spare space in a pot of cream. Generally we do try and measure our packaging so that we do not have wasted space inside of it.
Once the tube has been filled it is the run over a weighing machine which records the weight of the tube. If the tube is too heavy or too light the tube is moved off line and rejected. So the weight is a final check to make sure the correct amount of product is inside the tube.