User blog comment:DeathGodPrayer/Talk About What You Want!/@comment-162.202.198.72-20140119213531/@comment-16281343-20140119214240

There are other Dragons that showed the ability to shapeshift.
 * Yu-Long - turned into a horse
 * Crom Cruach - human form
 * Tannin - small dragon

That's what the author wanted. And also maybe to show the readers how Issei reacts to a child. Just like what happened in Volume 13, Extra Life. ~She tried to increase her breast size once.