Kazuya, at full power cannot even beat Kanaka's soul.
Real question is why does she look like a child?
She may have grown up, but she never really changed from how she was as a child. Her soul reflects that.
I just double checked that there isn't an incest tag listed. I guess they do keep skirting it by not going all the way. So, you're sister jacking you off, then teaching you to go down on her doesn't count. They don't show the latter, but he says he learned it from her when he does it to Kukuri and Kiriha. Now here, they show Kazuya and Taunade after they've clearly had spiritual intercourse, and they've been hinting who she really is. Spiritual intercourse in a place where they are both spirits. Kazuya was soul fucked by his mom, and will take his turn doing it later.