Top 10 most populates states in Nigeria:

Kano
Lagos
Kaduna
Katsina
Oyo
Rivers
Bauchi
Jigawa
Benue
Anambra

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

Based on 2006 Census figures

State	Population
Kano	9,383,682
Lagos	9,013,534
Kaduna	6,066,562
Katsina	5,792,578
Oyo	5,591,589
Rivers	5,185,400
Bauchi	4,676,465
Jigawa	4,348,649
Benue	4,219,244
Anambra	4,182,032
Borno	4,151,193
Delta	4,098,391
Niger	3,950,249
Imo	3,934,899
Akwa Ibom	3,920,208
Ogun	3,728,098
Sokoto	3,696,999
Ondo	3,441,024
Osun	3,423,535
Kogi	3,278,487
Zamfara	3,259,846
Enugu	3,257,298
Kebbi	3,238,628
Edo	3,218,332
Plateau	3,178,712
Adamawa	3,168,101
Cross River	2,888,966
Abia	2,833,999
Ekiti	2,384,212
Kwara	2,371,089
Gombe	2,353,879
Yobe	2,321,591
Taraba	2,300,736
Ebonyi	2,173,501
Nasarawa	1,863,275
Bayelsa	1,703,358
Federal Capital Territory	1,405,201
Total	140,003,542

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.