The size of the molecule differs. High molecular weight HA is a large molecule that cannot penetrate the skin. IT sits on the surface where it holds water (HA has an incredible ability to hold water) and makes it available to the skin. This significantly increases hydration of the upper layers of the skin – with all the resulting benefits attached to that.
Low Molecular weight HA is a smaller molecule that can penetrate the skin. It travels down to the dermis where it acts as a “water reservoir” holding water and increasing the hydration of the lowe layers of the skin. Together, these two provides multi-layer hydration to the skin.