Why is Luxembourg not included in the smallest country?

Luxembourg is included as one of the smallest countries in the world. According to the World Bank, the 10 smallest countries by land area are:

1. Vatican City (0.44 km²)

2. Monaco (2.02 km²)

3. Nauru (21.3 km²)

4. Tuvalu (26 km²)

5. San Marino (61 km²)

6. Liechtenstein (160 km²)

7. Marshall Islands (181 km²)

8. Saint Kitts and Nevis (261 km²)

9. Maldives (298 km²)

10. Luxembourg (2,586 km²)