Responses
It's 100% the mangaka's preference. In real life, any woman with breasts this big will have weight in her face, upper arms, and belly to match. I will hand it to the mangaka that he will add some of it. And on the specifics of Japanese women, Nonomiya-san is the norm and Kobuchi-san is closer to the ideal.
But that is part of the comedic affect. I can ignore the unrealistic parts with the fluff of the story. Personally, Nonomiya-san is my favorite character and her husband looks more like a foreigner/half Japanese due to his size and features. Okusan doesn't have ambiguity, which is why he can get away with it while she cannot.
Another woman with an unrealistic body type. Japanese women could never look like this no matter how hard they want us to believe it.