The convention of thin to thick in skincare is repeated everywhere without explanation. The reasoning is physical, and it determines how much of a product does anything at all.
What a serum is meant to be
A serum is a delivery vehicle carrying a relatively high concentration of active ingredients in a light base, usually water-based and low in oils and thickeners.
The lightness is deliberate. Fewer heavy ingredients means less competing for space on the skin and a thinner film that spreads and absorbs quickly.
A moisturiser has a different job. It contains emollients and occlusives that sit on the surface and slow the rate at which water evaporates away.
Occlusives block what follows them
The ingredients that make a moisturiser effective at holding water in also make it a partial barrier to anything applied afterwards.
A water-based serum applied over a rich cream has to cross an oily film that repels it. Much of it never reaches the skin surface evenly.
Reversing the order avoids that entirely. The serum meets bare skin, and the moisturiser then seals over the top without obstructing anything.
Molecular size sets the ceiling
Penetration depends heavily on how large a molecule is and whether it is soluble in water or oil. Only small molecules cross the outer layer in meaningful quantity.
Large molecules stay on the surface regardless of what is applied first. Some are designed that way, since a film-forming humectant is supposed to remain where it is.
This means layering order matters most for the actives that genuinely penetrate, and matters far less for surface-acting ingredients.
Why damp skin changes the result
Water is a carrier as well as an ingredient. Applying products to slightly damp skin increases how readily water-soluble ingredients spread and absorb.
The same dampness gives a moisturiser something to trap, which is the reasoning behind applying cream shortly after cleansing rather than much later.
Too much water dilutes an active and can reduce its effect, so the convention is damp rather than wet, and only for products designed for it.
Where the rule breaks down
Oil-based serums are an exception, since they behave more like the occlusive end of the range and are often best applied after a water-based cream.
Sunscreen is another. It needs to form a continuous even film, so it goes last regardless of texture, and nothing should be layered thickly over it.
The underlying principle is not a fixed hierarchy of product names but a simple question of what each layer would otherwise block.