You're probably confusing TioMAN (Nidhogg's consort/mother of his children) with TiAMAT child of Midgardsmnr; brood-sister of Nidhogg/Bahamut and consort of Bahamut
Speaking of Primal Bahamut, I am pretty sure one of the reason's why he was/is so powerful is due to the fact that he had thousands of years worth of prayers from those captured meracydian dragons + collecting energy from the Sun.
The real Bahamut, while strong, was defeated by the Allagan and it seems as if they defeated him pre-Omega weapon since it was the Primal Bahamut that was said to have been a threat to the Allagan
It really bad to measure a Great Dragon's strength based on Primal Bahamut. They're all strong, but not all-powerful. Just see Midgardsmnr vs a fraction of Garlean millitary; he was able to destroy one Dreadnaught at the cost of his then-adult body.
