The 10 least populated states of Nigeria are:

Federal Capital Territory (FCT), Abuja
Bayelsa
Nasarawa
Ebonyi
Taraba
Yobe
Gombe
Kwara
Ekiti
Abia

Full list of population size of Nigeria states. From smallest to largest

Based on 2006 Census figures

By the time that Nigeria got independence it was a federation of the three regions, namely:

Western
Eastern
Northern

Later on another region was added, the Mid-Western Region.

The Mid-Western Region was formed from the Western Region in 1966, and Lagos, the capital, was effectively governed as an unofficial fourth region outside the bounds of the Western Region.

This brought the number of regions to 4 as follows:

Western
Mid-Western
Western
Northern

Under the short-lived military government of Aguiyi-Ironsi, Nigeria was reorganized under a central government. Following a coup which resulted in Aguiyi-Ironsi’s deposition and assassination, Nigeria was reorganized as a federal country again with three of the regions being divided into newer entities and all first-level subdivisions being renamed as states.

Western Region was divided into:
Lagos
and Western states.

Eastern Region was divided into:
East-Central,
Rivers,
and South-Eastern states;

Northern Region was divided into:
Benue-Plateau,
Kano,
Kwara,
North-Central,
North-Eastern,
and North-Western (Sokoto) states

In 1976, six years after the end of the civil war, the states were further reorganized:

State boundaries and names were also reorganized.

Benue-Plateau state divided into:
Benue
and Plateau states.

East-Central state divided into:
Anambra
and Imo states.
Federal Capital Territory (Abuja) formed from parts of Niger and Plateau states.

North-Eastern state divided into:
Bauchi,
Borno,
and Gongola states.
Niger (Minna) state split from Sokoto.

Western state divided into:
Ogun,
Ondo,
and Oyo states

Akwa Ibom state created from Cross River;
Katsina state created from Kaduna

Abia State created from Imo;
Bendel State divided into Delta and Edo;
Enugu State created from Anambra;
Gongola State divided into Adamawa and Taraba;
Jigawa State created from Kano;
Kebbi State created from Sokoto;
Kogi State formed from parts of Benue and Kwara;
Osun State created from Oyo;
Yobe State created from Borno.

Bayelsa State created from Rivers;
Ebonyi State was formed from parts of Abia and Enugu;
Ekiti State created from Ondo;
Gombe State created from Bauchi;
Nasarawa State created from Plateau;
Zamfara State was created from Sokoto.