User blog comment:FairyRaven/Fairy Tail Chapter 435 & 436: The Cry of Victory & Memoirs, Review and Commentary/@comment-4933386-20150518131903/@comment-67.149.177.130-20150518154219

I believe it is that somehow Natsu's powers and memories were sealed inside the Book of E.N.D. I also believe because that happened, it had the same effect as Doriate's retrogression and Natsu's body became small again (explaining why he was a kid when he was with igneel).