Got a date tonight, she looks curvier in the pictures rather than fat but I don't know.
What is the difference between curvy and fat? 
You just stated it...a curvy chick or voluptous woman have curves in all the right places...no real "gut" , and has hips, ass, and breasts (your confusion may lie in the fact that you don't see too many Asian curvy women, and it's damn rare for any of them to have all three)
In terms of size...they can be a size 6, 8, or even a 14....I know of one chick who is 5'2 and 165, with NO GUT, NO jigglely thighs, or arms...she is solid as a oak desk
Fat chicks, might have a little shape, but they are fat in the gut, hips, ass, arms, thighs...you know it when you see it...