Most likely cause for Haruhi is macromastia. No, pills don't do that. Most are outright scams and don't even contain much herbs at all. Her poor jealous sister probably isn't even getting any significant herbs. Macromastia works by ultra high hormonal sensitivity, possibly from super high HGF and IGF-1 in the breasts (general growth factors). Maybe HGH too. Herbs work mostly by imitating hormones. Those with macromastia actually tend to have normal estrogen&etc. and are hypersensitive to their normal hormones. On top of that birth control, herbs or etc. are extremely effective on them typically adding several cup sizes. There's a middle ground too, where those with large but not giant breasts also tend to be more responsive to these (but not as much as those with full blown macromastia).
Based on theory, animal studies, and very limited human use yes we probably could easily induce macromastia in humans with a series of injections. IGF-1, HGF, HGH, estrogen, progesterone and a testosterone blocker. At least 8 cup sizes and probably several more than that. Would also be a huge risk for breast cancer, which is probably why they aren't so eager to try it out. There are plenty of super giant breasted lab animals out there though. Herbs tend to be weaker than pure hormone and may contain some anti-cancer compounds too, which is probably why we don't get a thousand breast cancer horror stories like we do with those who inject hormones. So seriously, don't actually inject hormones. People really do get cancer from this. Doctor monitored birth control, herbs, ok. Severely limited IGF-1 and progesterone are probably ok. Illegal high estrogen, high IGF-1/HGH, etc., nononononono.
Haruhi probably has a higher than normal breast cancer risk but it still probably isn't a huge risk. And there's not much she can do about that risk anyway, so may as well enjoy
... and probably get a reduction for convenience when she gets older and the fame & pics aren't so great.