I have made the observation that Amano's character designs tend to be very tiny, whereas Nomura's are more of an 'athletic' body type.

I would never define Maria, Aerith, Rinoa, or Yuna as "curvy"
To me they just looked a little more voluptous than the average FF heroine. Maybe my eyesight is failing me.

Even most of the male body weights given are incredibly light. I'm fairly involved in sport and I can tell you that they are extremely light as measured against the standards of high performance athletes (which I would assume all the protagonists would be).
A very well-made point, the males tend to be just as tiny, or very large (e.g. Ward from VIII or Snow from XIII)!

But then again, it is a fantasy game; the characters, no matter how realistically portrayed, are still not real people and never will be, so how big or small they are shouldn't really be an issue.