Actually HRT can produce significant changes in a short time frame, for certain people, it all depends on the body. She does not start as completely flat chested either, as she tells us when she comes out that she's been on it for 6 months. Hence the use of a hoodie all the time and when she hugs her friend and he remarks, she talks about the light binding she does. The sliders from Chapter 2 to the end of chapter 3 did not change. The slight modification in chapter 4, is so tiny it is barely noticeable. Hehe, I look at her naked body everyday, take it from an expert here
Clothing is a massive aspect in how you can go from not showing to showing. There is a world of difference between the baggy jumpers and hoodies she use to wear to the tight, clingy stuff she's now wearing.