Fine hair is defined by strand diameter. Each individual strand is narrower than average, which means less structural mass, faster oil transit from scalp to tip, and more sensitivity to product weight. Fine hair doesn't need less product — it needs the right active ingredients at the right molecular weight, applied in the right place.
Professional volumizing formulas use low-molecular-weight polymers and specialised technologies: Bumble's thickening complex, Living Proof's PolyfluoroEster Technology, Kérastase Volumifique's biomimetic protein.