The study of 117,000 women in Denmark found that those who were chubby at birth but tall and lean at 14 were more likely to develop the disease.
"Something very early on in life plays a role in risk of breast cancer," said lead researcher Dr. Mads Melbye, professor of epidemiology at the State Serum Institute in Copenhagen.
"No one knows really what," he said, but theories include differences in levels of hormones that influence growth and genetic variations that dictate people's size.
Most studies of women have found that tall ones have an increased risk of breast cancer, that heavy ones have a higher risk of the disease after menopause, and that lean ones have a higher risk before menopause and a reduced risk after.
"We as researchers need to focus earlier in life to disentangle what really matters," Melbye said.
The study was reported in Thursday's New England Journal of Medicine.
The researchers found that the taller a girl was at 14, roughly the end of puberty, the higher her chance of later developing breast cancer. For example, a girl 5 foot, 6 inches tall at 14 had about a 50 percent higher risk of later developing breast cancer, compared with one who was just under 5 feet at 14.
Babies who weighed 8.8 pounds at birth had a 17 percent higher risk of later breast cancer than ones who were only 5½ pounds. Each additional 2 pounds over 5½ boosted risk by 10 percent.
The lower a girl's body mass index - a measure of weight relative to height - at age 14, the higher her risk of breast cancer, echoing findings about the risk in pre-menopausal women.
The Danish researchers also found the younger a girl has her peak growth period, the higher her risk of later breast cancer.
"It is confirming things that people had long suspected," said Dr. William Hait, director of the Cancer Institute of New Jersey.
He said factors that influence initial development of breast tissue in the womb and how early a girl's breasts develop, as well as what a pregnant woman eats and drinks and her daughter's diet during childhood, appear to affect the risk of breast cancer.
One surprise was that, after adjusting for the other risk factors, the researchers found that the age at which a girl began menstruating did not influence her risk of breast cancer. It has long been dogma among doctors that the earlier menstruation begins and the later menopause starts, the higher the risk of breast cancer.
Breast cancer will kill about 40,600 American women this year, according to the Centers for Disease Control and Prevention. It is the second most commonly diagnosed cancer in women, after skin cancer.
Hait and Dr. Yelena Novik, a breast cancer specialist at New York University Cancer Center said the study was particularly strong because of the huge number of girls studied and the detailed school and adult health records available in Denmark. But Novik said it would have been helpful if the study had examined the women's weight and height as adults and whether they had a family history of breast cancer.
In an accompanying editorial, Drs. Karin B. Michels and Walter C. Willett of Harvard Medical School wrote that studies in animals and of girls exposed to radiation from atomic explosions indicate that the time between conception and the development of mammary glands may be when breast cells are most vulnerable to cancer-causing influences, such as from substances in food.
They wrote that there has been a steady increase in both the rate of breast cancer and women's average height in Japan over the past five decades, probably because of a change in diet. Likewise, breast cancer rates have been rising at the same time the world population has become taller over the past century.
Identifying the exact biological mechanisms at work could eventually produce strategies for reducing the risk of breast cancer, they wrote.
Melbye and the other doctors said pregnant women and young girls should not try to adjust their growth patterns based solely on the Danish findings.
"If you do that, you may actually do harm in another area," Melbye said, such as by fattening up in adolescence and developing a weight problem that could lead to heart disease and other obesity-related ailments.
By Linda A. Johnson