If a boy decides to leave you or not show interest in you purely because your breast size is not as big as he wants... he is no more than a dick!
You should not put too much into this, just let it be naturally and focus more on other factors in a relationship. There can be alot more things that you can have control in, compared to breast size... for example... 品德 修養 學識 禮儀 or even how you dress, apply make up, etc.
Choose the guy who knows how to appreciate you for who you are... not cause of your boobs!