They don't they expand.

uhm.. is your freezer broken?

I don't know how they shrink. Maybe your freezer is not set on a low-enough temperature that the water melts and then when someone opens the freezer and slams it shut, the water spills, and then when you open it to check on the ice cubes, they look tinier.

they do over time. i know what you mean. they may evaporate or something, not 100% sure. but after reading the other answers, i thought i would let you know, i've seen it too.

The ice sublimes. The is where the solid moves to the gas phase with no liquid phase between. It also occurs from the temp changes when the door is open and the ice may melt ever so slightly (not visible to the eye) and this water evaporates. It does take a long time for significant shrinkage to occur though. :)

It is called dehydration because of automatic defrost.

It's dehydration from the air circulation. It seems to take several weeks for them to shrink noticeably.

The air circulating is how the automatic defrost works when the fan comes on.

3-10-2017 Ice will evaporate. It goes from solid to vapor without going through the liquid phase. When that happens to food it is called "freezer burn".

Sublimation: phase change from solid to gas.------ '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.' Source: https://www.reference.com/science/ice-cubes-shrink-freezer-212dca256db27ac2