Ultrastable glasses produced by vapor deposition exhibit properties consistent with glasses that have been aged for thousands of years or more. These materials' properties are believed to emerge from the presence of a mobile layer at the surface of supercooled liquids that allows access to lower-energy states. However, the precise mechanism by which this enhanced. mobility is translated into ultrastable glass behavior remains incompletely understood. Here we show that enhanced densities and stabilities consistent with ultrastable glasses specifically can emerge as a result of a mismatch in the length scales of thermodynamic and dynamic gradients at the surfaces of equilibrium supercooled liquids. In particular, ultrastable glass properties can be understood within a three-layer model of the interface in which a "facilitated layer" intermediate between the surface and bulk exhibits bulk-like liquid-state density but suppressed T-g. This mismatch in length-scale has previously been correlated with the scale of cooperative rearrangements in the supercooled state, suggesting that ultrastable glasses may be a direct consequence of the cooperative nature of dynamics in equilibrium supercooled liquids.