Ice sometimes shrinks inside of a frost-free freezer because of a process known as sublimation. According to the University of California, Davis, sublimation occurs when a solid, such as ice, quickly vaporizes into a gas without first changing into a liquid.
For sublimation to occur, the air surrounding the ice must become energized, transfer heat and excite the molecules in the ice enough to reach its maximum heat capacity in its solid and gas states. A maximum heat capacity is necessary to break the bonds holding the ice together and quickly heat it to a gaseous state.
For the most part, this phenomenon occurs in frost-free freezers because they frequently heat the air, which creates an environment for sublimation to occur. According to HowStuffWorks, frost-free freezers have a heating coil wrapped among its freezer coils. Approximately every six hours, the freezer turns on its heating coils to melt the ice that builds on the freezer coils. When the temperature begins to rise above 32 F, or zero C, the timer shuts off the heater. During this process, however, the hot air generated by the heaters warms the air surrounding the ice. This makes the top layer of ice hot enough to quickly sublimate, or change from ice to gas. Because some of the ice has changed to a gaseous state, the ice loses volume or shrinks to a smaller size.