a) Breasts that have not been enhanced by plastic surgery.
b) Natural, albeit smaller breasts.
Popularized in the mid-2000s when American actress Lindsay Lohan was rumored to have undergone breast augmentation surgery. Lohan vehemently denied the charges, while her declining reputation became constant tabloid fodder.
This was in contrast to her old "rival" Hilary Duff, who still maintained a squeaky clean public image and had a visibly smaller bust line compared to Lohan. Hence the term Duffs was born.