Undersizing an expansion tank can lead to inadequate pressure relief, resulting in excessive pressure buildup in the system. This can cause damage to the water heater, pipes, and fittings, and may even lead to catastrophic failures. Additionally, undersizing can lead to frequent relief valve operation, which can result in water waste and increased energy consumption. In extreme cases, undersizing can cause the system to become over-pressurized, leading to a rupture or explosion.