Kubo said this in response to the interviewers question.

Interviewer: Kubo-sensei, just why is Yammy so strong? I never suspected he would be so amazing to be honest.

Kubo Tite: Haha, I bet It's pretty weird huh. Well I don't want to spoil too much but I'll let you in and the fans in on some interesting information. Yammy is the most unique and strongest arrancar in existence. This is because he was capable of reaching a higher class than Vasto Lorde who have human form but limited power. Yammy however is in his own class that can summon infinite power and utilize the potential of that destructive power as he pleases. All Vasto Lorde tend to have a very humanoid form when they release their zanpakutou but Yammy has the form of a giant. The more reiatsu he builds up, the more power he gains and thus his size and destructive power rises as well. He's also been alive for nearly 5000 years and that makes him the oldest arrancar in history. The reason he is ranked "0" is due to the fact that he has an absense of a specific level of power. In his unreleased form, he had a hard time against Ichigo and Urahara but when he realeases and begins to get serious, he's able to even threaten the Royal Guard.

Interviewer: That's shocking! I always thought Baraggan was the oldest of the group but Yammy? This is very shocking sensei. May I ask if you had planned this from the start?

Kubo Tite: It's all going just as planned. I've known about Yammy's entire power set and history from the moment I came up with his design. I'll also give you a little bit more information as well. Yammy actually has an interesting history with Kirio Hikifune haha.

Interviewer: The former 12th division captain that went to the Royal Guard? That sounds interesting *laughter*

Kubo Tite: What I'm about to say is somewhat of a big spoiler but I'll give the fans one final big treat to look forward to. The truth is that hollow and shinigami have been at war for 1000's of years. One moment during a hollow invasion, Yammy and Hikifune fought in Hueco Mundo and as they fought, they began to understand eachother. Despite being different begins, they all possess some sort of emotions. In the end, Yammy and Hikifune began to become friends in secret and eventually that friendship would turn into love. The final thing I'll mention is that they both had a child before Hikifune was forced to depart from Yammy and that child was hidden in the real world.

Interviewer: Wow *laughter*. What type of child was it?

Kubo Tite: Well lets just say that he ended up growing up in Mexico and inherited his fathers "punching strength".

Interviewer: Are you talking about Sado Yasatura?

Kubo Tite: I made it obvious didn't I *laughter*. Sado was left in the care of Oscar Joaquín de la Rosa who Sado would refer to as his grandfather by Hikifune. This was to insure he could have a normal life in the prosperous country of Mexico. He was also lied to as a child when they said his parents died. It was so he wouldn't be confused about his true origins. In any case, this is why Sado's powers are very similar to hollows. Unlike Ichigo who became a Vizard after his birth, Sado was born as a mix between Shinigami and hollow and as a result, his powers combined into a completely new form. You could say his two arms "Brazo Izquierda del Diablo" and "Brazo Derecha de Gigante" are similar to zanpakutou in a way. He could only harness the true power of his arm by realizing their true power and names and utilizing them. Thanks to the Hougyouku, Sado's powers were fully awakened and he was allowed to develop them as he pleased.

Interviewer: Unbelievable. I'm really looking forward to seeing how all this plays out. Does Yammy know he's been attacking his own son?

Kubo Tite: He hasn't realized it yet....but the future can be very interesting *laughter*