If we consider the manga as canon, her larger boobs would actually be more accurate
The manga even specifically jokes about her tracksuit hiding how big her chest is, so it's interesting that the anime completely removed any hint of a bust even when wearing outfits that would show it.
Pretty much no one in the show though really has a bust, so it's more of a style choice for everyone. The anime does recognize Hitori's bust size though, particularly in episode 11 you have short segments like 7:15 where when they do draw some definition to her design there definitely is a well defined bust. Then there is the short part where she is depicted in a bikini, and she definitely is shown with a large bust. Of course as they're either stylized or imagined scenes it does leave room that the depictions are exaggerated or fake. Especially as after the bikini they have her depicted in body tape and she's definitely back down in bust size.
I mean she is supposed to be a high schooler 16 at max. The show took a different direction to make her more of a relatable homie.
breasts dont start growing at high school, anime tends to pretend a 18yo is supposed to have bigger breasts than a 14yo but breast growth in that age window is most often much less significant than below 14. I remember back in middle school knowing a few girls with basically the same breast size as Hitori when they were just 14, and i'm sure most people who remember a little bit their school years can say the same