Rather than an energy shield, heavy shields are physical composites of metal or plastic. They are much larger and more cumbersome, but they offer more protection than their smaller counterparts. Its cumbersome size, however, makes it unwieldy, and requires the use of a one-handed weapon with the light property in the other hand.
Small creatures have disadvantage on attack rolls while wielding a heavy shield. A heavy shield’s size and bulk make it too large for a Small creature to use effectively.