In Naruto, tsunade boobs can definitely be said to be the biggest, but when she was young, her chest was not big at all, and she was even mocked by Jiraiya for being flat-chested. So why did tsunade boobs change so much?
First, it may be due to Tsunade's own abilities. We all know that Tsunade's medical ninjutsu is considered the strongest in the ninja world, and she also has a very powerful technique called Yin Seal: Release, which can not only improve the body's functions through precise chakra control and achieve eternal youth, but also automatically regenerate the body at high speed when injured.
Tsunade's advanced medical ninjutsu represents her deep understanding of the human body, and since the Yin Seal: Release technique can enhance the body's functions and accelerate cell division to automatically heal injuries, why not combine the two to stimulate the growth of chest cells?
Second, it may be due to genetics. We all know that the founder of the Senju clan is Asura, the son of the Sage of Six Paths, and the Sage's daughter-in-law, Kaguya Otsutsuki, ate the fruit of the God Tree. Tsunade's grandfather, Hashirama Senju, as a direct descendant of Kaguya Otsutsuki, and with Asura's chakra residing within him, his genes are definitely top-notch and can be said to be unparalleled in the entire ninja world. Moreover, Hashirama's wife was Mito Uzumaki, a distant relative of the Uzumaki clan, so the genes passed down to Tsunade are arguably the most advanced in the ninja world. With such advanced genetics, it's reasonable to assume that Tsunade has a large chest!
Third, it's possible that tsunade boobs is fake. According to the set-up book, Tsunade's height is 163.1cm and her weight is 48.9kg. Judging from these data alone, Tsunade should be thin. As a ninja skilled in taijutsu, Tsunade's muscles must be more developed than an average person, and her body fat percentage naturally should be lower, making her weight naturally heavier than an ordinary person of the same height. However, in fact, Tsunade's weight is even lighter than an ordinary person's. According to the saying "If the weight is not over 100, then it's either flat-chested or short," Tsunade's weight is not over 100, but she has such a broad chest, which is obviously unreasonable. Of course, the ninja world is inherently unscientific, and tsunade boobs is unlikely to be fake, but based on her height and weight, I remain skeptical unless I can touch it!
Finally, it may be due to the author's reasons. Generally speaking, most female characters that appear in literary works are very beautiful unless there is a special plot requirement, and Naruto is no exception. As a popular manga, it's normal to have some busty female characters besides Tsunade, such as Hinata Hyuga, Samui, and Mei Terumi.