We did human observer experiments using a hybrid image technique to determine the variation of tumor contrast thresholds for detection as a function of tumor sizes. This was done with both mammographic backgrounds and filtered noise with the same power spectra. We obtained the very surprising result that contrast had to be increased as lesion size increased to maintain contrast detectability. All previous investigations with white noise, radiographic and CT imaging system noise have shown the opposite effect. We compared human results to predictions of a number of observer models and found fairly good qualitative agreement. However we found that human performance was better than what would be expected if mammographic structure was assumed to be pure noise. This disagreement can be accounted for by using a simple scaling correction factor.