I'm going with my headcanon they where equal when the fight started, but after 10 days of fighting Akainu finally unlocked the power no one ever did before him, awakening a logia, this gave him the advantage to win the fight.
This would also put him above the other admirals with a fair margin, making him still a relevant battle strenght wise against Luffy
This would also put him above the other admirals with a fair margin, making him still a relevant battle strenght wise against Luffy