If you're a real person who wants to dramatically increase your muscle mass, you have to follow a strict diet and exercise program. Depending on what kinds of results you're looking for and what your physical condition is like when you start, the process could take months or even years.
But anime characters aren't necessarily subject to the limits of the human body. For anime characters who can temporarily bulk up, a totally jacked body is only minutes away. Keeping their pumped up physique is another story, however - in most cases, these characters become small again soon after they change. All Might of My Hero Academia used to be consistently ripped, but an injury made it impossible for him to maintain his muscle form. Meanwhile, Escanor of The Seven Deadly Sins undergoes a massive shift in physique every day.
Which of these characters has the most interesting reason for transforming?
Once upon a time, All Might was jacked 24 hours a day. But after a grievious injury sustained while fighting All For One, he withered away until he was practically a skeleton. However, he was still able to assume his muscular form for short periods of time. When Izuku first meets him, he's able to do it enough to keep his injury a secret, but as the series wears on, this ability decreases. When he fights All For One in one of the greatest showdowns in the history of the series, he finally loses his muscle form all together.
Depending on what time it is when you meet him, Escanor might be totally juiced, or he might be totally scrawny. Thanks to an ability called Sunshine, he spends his nights as a weak, timid man who looks like he could be knocked over by a light breeze, and his days a confident, muscle-bound man who could easily benchpress someone the size of his previous self. How he hasn't had a heart attack from all these rapid physical changes is anyone's guess.
Master Roshi might look like a frail old man, but you'd better not underestimate him. When he uses a power-up called Max Power, he swells to several times his normal size, both in height and in muscle mass. Despite being much larger than usual, he's every bit as agile as he usually is - and a hell of a lot stronger. Max Power allows him to use all of his techniques to their full potential - one notable example is Max Power Kamehameha, which makes the regular energy attack look weak by comparison.
Kale's base form is actually pretty unusual as far as Saiyans go. Not only is she small and weak looking, she also lacks the confidence that is characteristic of her race. At first, the only way she can believe in herself is with the advice and support of her mentor Caulifla. But when she accesses a mode called Berserker Super Saiyan, she not only becomes outlandishly shredded, her personality does a complete 180. Her timid nature completely disappears, and is replaced with serious swagger and almost uncontrollable aggression and rage. Gaining control over her powers in this state is a serious challenge.