Xcorsx nailed it pretty much, you cant base curvy and fat on dress sizes ect because those pretty much vary from individual to individual based on how tall they are,exercise,diet,genetics ect
Someone may be the exact same height and weight as someone else and still look completely different due to how and where they carry their weight.
Traditionally the whole 3 digits(eg 34-24-36) have been a very good association of how curvy someone is, the greater the disparity between the waist size and bust/hip size the more curvacious they are.
I know girls who weigh less than 50kg who I would consider curvy, I know girls who weigh more than 70kg who I would consider curvy, I also know girls who weigh 55kg who I would consider fat, depends entirely on the individual.