User blog comment:IamJakuhoRaikoben/Chapter 471: Until the Battle is Done, Review./@comment-5303512-20160202131244

I am starting think that human form Acnologia is his strongest form and that the Dragon Form is just to intimidate others.

Thinking about it rationally, Dragon Slayers who use to much DSM will turn into Dragons themselves, it is one of the many taboos, limitations, restrictions and side effects of Lost Magics because they are unfitted compared to other magics. It works just like Survival of the Fittest, only the best may survive.

Back to Dragon Slayers turning into Dragons, this cannot possibly make the Dragon Slayers stronger, more like give them a great big Achilles' Heel for other properly trained Dragon Slayers.