Conversion to estrogen is definitely something that happens, though it generally happens when testosterone levels are excessively high.
Gynecomastia is a complex condition that can have several potential causes.
But I can answer your question about whether your breasts will get larger. The short and simple answer is yes. I also had gynecomastia during my teen years (and of course they never went away in my adult years). Just like you, I had 38B breasts before transitioning.
You'll definitely have some further breast development. How much is impossible to predict. Several factors are involved. Age, activity level, general health, diet, lifestyle factors (such as smoking and alcohol consumption), and genetics. I was 36 when I started HRT, am very athletic (which has an impact on your body's utilization of cholesterol derivatives such as estradiol, progesterone, and testosterone), I had never smoked or drank alcohol, and the women on both sides of my family tended to have larger breasts. I am currently 38D or 36DD, depending on the bra and how it's cut.
Not knowing your personal health, lifestyle, and family genetics, the only thing I can say is that age could make the effects on breast development less pronounced. What that will equate to in terms of cup volume is hard to predict. But since you're already a B, you will definitely be at least a B.