User blog comment:IamJakuhoRaikoben/Chapter 415: And It Shall Become Your Will To Live, Review./@comment-7418318-20150110171453/@comment-7418318-20150110195930

Considering how Igneel was able to fight equally with Acnologia, maybe it is a matter of stamina rather than strength that was halfed.

What I mean is that in life Igneel might have been just as strong as in this half-dead state at the beginning of the fight, but could have continued to fight for much longer.