The United States (like Australia) is not a unitary State with a single sovereignty deriving from a prior or current monarchy like most European States.
Each US State is a sovereign entity which existed prior to joining the United States and whose rights are protected in the US Constitution and electoral system. States can enter or leave this Union.
Thus there is no promise in the US Constitution that every US citizen's vote will be worth the same. Indeed in the US Senate each state gets two Senators regardless of size.
The issue derives from the nature of sovereignty.