http://www.somali-jna.org/downloads/ACFA9.pdf
I print-screened this table they provided in their documents showing the 18 regions of Somalia and estimation of each regions population throughout a period of 15 years. (1995-2015)
Somalia population was roughly 4.9 million in 1975 and in 1985 it was roughly 6.1 million. Don't forget Somalia has the highest fertility rate in the world but also the highest mortality rate.
According to the UNDP estimation the top 5 regions with the highest population in Somalia come in this order:
1. Banaadir/Mogadishu
2. Bay
3. Lower Shabelle
4. Middle Shabelle
5. Gedo
the top 5 regions with the lowest population came in this order:
1. Sool
2. Nugaal
3. Awdal
4.Middle Jubba
5.Sanaag
The remaining 8 regions were average population.
Here is the figure from UNDP:

UNDP has predicated Somalia population in 2015 to be roughly 12 Million. Afcourse Somalia is a unpredictable country due to the ongoing fighting and nomadic movements.