600cc seems like a big implant just because of the number "600", but how big they look on someone depends on so many different factors. The most important factors include height and weight, amount of breast tissue, how firm the breast tissue is, characteristics of the skin, and width of your breast base diameter and the shape of your chest wall and rib cage.
With that said, there are certain bits of information that you provided that would steer me against recommending a 600cc implant. You are 5'3" and 125 lbs. You describe yourself as athletic. 600cc implants should look very large on someone of your height and your weight. 600cc implants on most women of your frame generally don't make them look athletic. They do make you look very full though. So, it all depends on your goals and how big you want to look ultimately. Also, with a 45 lbs weight loss, your skin may be not as elastic and tight and so, the weight of 600cc implants may make your breasts sag over time. Large implants that sag on a small framed woman can also make them look heavier.
So in summary, 600cc may be too big, but I would recommend seeing a board certified plastic surgeon and be examined to see what implant range may give you the look that you want.