She doesn't?
She has the body of a strong, physical fighter who can go toe-to-toe (considering the setting of the game) with demons, celestial beings, and other badasses.
Oh, do you mean traditionally feminine? Well sorry, but that would be inappropriate! You see, people have various body shapes based on genetics, diet, and exercise! A lot of the more "traditional" feminine champions, including the often cited
are like that for a reason! You see, most of them use magic or weaponry for ranged combat, and thus are unused to physical brawling that Illoai would be. Constant physical exertion = more muscles.
Secondly, using those champions as an example, most of them actually have justified reasons for their "traditional feminine" appearance. Sona is a musician, and thus dresses and upholds herself appropriate for that fashion. Ahri began her more sentient life seducing and absorbing the lives of men, and uses magic and trickery over brute force, so the more "femme fatale" appearance is actually appropriate for the battleground. Zyra has a similar backstory, and purposely shaped her form to be tempting toward men! Miss Fortune probably has some skill in the seduction department, but considering how badass she is probably gives 0 fucks.
You see, these "traditionally feminine" women are like that for a reason, and would not be able to be like Illaoi just like Illaoi would not be like them. Armor and proper protection would probably hinder the four cited champions' abilities, and honestly would be useless, whereas Illaoi cannot use the same kind of magic, weaponry, or ranged abilities they can!
tl;dr: When you constantly physically exert yourself, instead of relying on magic or guns, you tend to build up muscle.