Hi Misszee,
Excellent and fair question - hard to give a precise answer.
In GENERAL, it's about 150-200 grams per bra cup size, but it's less at the smaller end, and more at the larger end. In other words, it's less than that to go from a AA to an A, and more from an F to a G (or the other way around). Since a DD is an E, and a DDD is also called an F, you're basically two bra cup sizes above a D. Depending on your height and weight and your frame, my guess is that a 500 gram reduction would take you down about 2-3 bra cup sizes. Weight also depends on the density of the breasts. For young women and/or dense breast tissue, 100 grams will be a much smaller amount (ie, will not reduce the cup size as much as) than for an older woman and/or fattier breast tissue (nothing personal, of course....)
So I cannot say for sure without seeing a photo and/or being able to examine you and, in fact, it's hard to be exact even if those conditions are met.
Ideally, you should have a frank and open discussion with your plastic surgeon about what you want and then be comfortable accepting a range of cup sizes, as long as you're reduced, lifted and the shape is appropriate for your frame.
I hope that this helps, and good luck,
Dr. E