After Kazuha and Naoto have sex that first time because of the ruler’s gift making them obey. So that Naoto and Kazuha gets blackmailed and Naoto feels guilty as he is now the one responsible for taking Kazuha’s virginity. Kazuha is altered just enough, most likely to be obedient or not question her own actions later.
Later on, the ruler and Naoto end up talking with each other by chance, Naoto notices he’s always in the innkeeper husband appearance whenever he is about to have sex with a woman. He basically tells the ruler that being too big is bad as it’ll cause scars and injuries to women, that he should go for more normal size for women. And to treat women better. The ruler sorta takes the advice.
Later we see Kazuha is sitting on the ruler’s bed taking off her clothes. She thinks to herself that the ruler isn’t ugly or handsome, just average. The ruler is thinking to himself this is the first time he’s had sex with a woman he hasn’t changed completely(basically made them a whore) just altered a bit. And the fact she’s a Japanese girl rather than one of the “NPCs”. He finds it exciting as he’ll be having sex with a girl from Earth and that she completely hates him too.
When the ruler undresses and reveals his thing, Kazuha makes a momentary confused look on her face when seeing the ruler’s thing and she averts her eyes a bit. The look the ruler interpreted as “it’s only that big?” As if she was comparing his to Naoto’s.
Basically cause of his fragile ego and his over reliance of his gift. He completely ignores Naoto’s advice and makes his you know what even bigger and rams it into Kazuha without any foreplay. The ruler thinks that size matters more and he refuses to lose to Naoto. He believes that he’ll outdo Naoto when it comes to sex. This is where the chapter ends with the ruler having sex with Kazuha finally, since the ruler didn’t use his gift to completely change Kazuha.
In the previous WN chapter, Kazuha had complimented Naoto’s sincere actions when they had sex. That he was true to his word that he would be gentle with her and let her feel great pleasure.
