What I know about the Niger Delta fight was mainly because of marginalization, the Niger Delta region was the smallest region in Nigeria and as at then the government where not given them attention mean while the oil that is feeding the nation was coming from there and so the Niger Delta people said no they will not accept this marginalization and they started fighting by destroying pipe lines where oil was pass through, they may have now requested for there own country so they can stay alone as the first was going on, but the main reason for the fight was because of marginalization.
Well if you say Niger Delta, you are for sure talking about 6 states now (rivers, Delta, akwa ibom, edo, bayelsa and cross rivers). They were never small in the manner you are looking at it, sure population has increased everywhere.
So looking at the marginalization you pointed, it was basically the ijaw area of the Niger Delta which encompasses part of rivers, bayelsa and I think Delta State. I guess you saying that Niger Delta "was the smallest region in Nigeria" means they are now big... Or better I don't really understand that..
Niger Delta is the whole of south-south and ijaw is just a part of the ethnic groups in the Niger Delta.
As of the oil even back then, it was not just the ijaw region of Niger delta that produced oil, I guess they are not even producing up to half of the total production right now..