The rules for considering something an island aren't clear.
Sodertorn is included being an artificial island made thanks to cutting off the peninsula with canals but Peloponnese (more than a 1 million population) is not despite the same origin.
IJsselmonde is included being a river island but Zitny ostrov (more than 200 thousand population) is not despite the same origin.
Södertörn is probably a natural island. Before the Södertälje canal was made, there was a small sound of water at the same place, so Södertörn was already an island.
Sodertorn is included being an artificial island made thanks to cutting off the peninsula with canals but Peloponnese (more than a 1 million population) is not despite the same origin.
IJsselmonde is included being a river island but Zitny ostrov (more than 200 thousand population) is not despite the same origin.