Why skin starts to look less full
Most people notice it as a look rather than a symptom. Skin that used to look plump looks flatter, and it feels tighter and drier by the end of the day.
It is mostly about water
Skin that is holding water looks fuller and feels softer. Skin that is not looks flat and feels tight. That is why the same face can look different on two mornings with no other change.
What a surface serum does
Humectants like glycerin and hyaluronic acid draw water into the surface and hold it there. That changes how skin looks and feels while you keep using them. It is upkeep, not a one time fix, and anybody selling it as a one time fix is selling you something else.
What it does not do
It does not change the structure of your skin, and nothing you can buy in a bottle in New Zealand without a prescription claims to honestly. Be careful with any product that puts a number on how many years it will take off.