Isn't that already a thing though? Surgery to reduce breast size and implants to increase breast size already exist...

Thats different as thats adults (well was) if there was a pill a developing child could take that would dictate the growth of theirs, thats a totally different bucket of fish and getting close to designer baby territory

You don't think it is cruel to force girls to have big breasts despite how uncomfortable they can be?

you think parents or children as young as 10 should be able to decide what size breasts they have? you think that would turn out well?