Examining 25 years of cancer records nationwide, researchers concluded that smaller tumor size accounted for 61 percent of the improvement in survival when cancer had not spread beyond the breast, and 28 percent when it had spread just a little.
For women 65 and older with early-stage tumors, the most common scenario, the shift in size accounted for virtually all of the improvement in survival.
"We don't in any way want to diminish the benefits we've seen from advances in treatment because they've been enormous," said lead researcher Elena Elkin. "But not all of the improvement in survival is due to treatment when important characteristics like size have also changed over time."
The study wasn't designed to determine the value of mammograms or treatments. But it implies much about the value of early detection. Mammograms have increased dramatically in recent years and are the chief reason that cancers today are detected sooner and smaller than they were in years past.
"This really helps to show the importance of screening," said Debbie Saslow, who heads breast cancer research at the American Cancer Society. "In addition to finding more small tumors, we're also finding less big tumors."
Saslow had no role in the study, which was being published Monday online by the society's journal, Cancer, and will be in its Sept. 15 print edition.
It was conducted by doctors at Memorial Sloan-Kettering Cancer Center in New York and used a federal government's database that includes nine cancer registries covering 10 percent of the U.S. population. More than 265,000 breast tumors were analyzed.
Breast cancer is the most common cancer in American women. An estimated 211,240 new cases and 40,400 deaths from it are expected this year.
Survival has increased, but experts have argued over how much of that is because of better drugs or tumors being found at earlier stages. Two-thirds of breast cancers today are diagnosed at the local stage, when they're still confined to the breast; in the 1970s, only half were.
However, this is the largest study in American women to look at size within those stages.