Honestly, i think a lot of people have issues with their own breasts, so assume everyone else does. Since yours are so different to theirs, maybe they are asking such questions to try to convince themselves grass isn't always greener and that you're as unhappy as they are. They may be unhappy or insecure about the fact they do have large breasts.
Me myself, my insecurities are that my right breast is quite a lot bigger than my left. Noticeably bigger, as in at least a cup size and a half size difference. They are also completely different shapes, with the right being a fuller, rounder shape. I could be happy if both my breasts were like my left breast, and i'd be incredibly happy if both were like my right breast. As it goes though, i'm not about to go through surgery to alter them.
Another issue i have is congenital symmastia. My breasts don't have a gap between them, rather they are joined together. I just have a crease goes down like if you fold a page in half with one side being my left breast, one being my right. The crease isn't central either. Its not quite as extreme but is the same issue some women suffer post implants when they're put in too close together. Sadly even if i had corrective surgery on this, it would likely fail or revert, so no point even attempting it.
That bring said, i wouldn't ever assume any other woman with differently sized or shaped breasts, or symmastia was unhappy with how her breasts are. I would absolutely NEVER comment on anyones body.