User blog comment:Wrath022/Chapter 516: The Truth About Enchantment, Review/@comment-4507125-20161231163540/@comment-1359322-20170101020951

I think it's more to the usage of the magic itself. Acnologia slayed so many Dragons, his transformation probably became drastic because he used the DS magic more than the others. Then you have Irene who loves Dragon, i doubt she'll randomly make use of the DS magic to kill Dragon like Acnologia would. Though in the end we don't know what happened to the other Dragon Slayers 400 years back nor do we know the real requirement of fully transforming into a Dragon, so everything we say now is purely speculation.