The surprising conclusion is made by Karolinska Institutet researchers who looked at data from 2,818 Swedish breast cancer patients and 3,111 healthy counterparts.
"Large body type at age seven years was associated with a decreased risk of post-menopausal breast cancer," said lead researcher Jingmei Li.
"It appears counterintuitive that a large body size during childhood can reduce breast cancer risk, because a large birth weight and a high adult BMI have been shown to elevate breast cancer," Li added.
"There remain unanswered questions on mechanisms driving this protective effect."
BMI means body mass index, a measure of fat.
A girl's BMI at the age of seven also dictates the risk of so-called oestrogen receptive negative tumours, where the outcome is often less favourable than other cancer types, said the study.
The paper is published online in an open-access, peer-reviewed journal, Breast Cancer Research.
Separately, in a paper published on Wednesday in the journal Nature, scientists drew a map of 20 genetic mutations behind a highly lethal, fast-spreading form of breast cancer.
So-called "triple negative" breast cancer disproportionately affects younger women and those who are African-American.
The gene profile came from a 44-year-old African-American woman who was killed within months after breast cancer spread to her brain.
The work adds to knowledge about fast-track breast cancer and throws up new drug targets for women diagnosed with "triple negative" tumours, said Elaine Mardis, co-director of the Genome Center at Washington University.
"We are getting an intimate look at the lethal spread of a breast cancer, which is now possible because we can sequence entire genomes quickly at a reasonable cost," said Mardis.
The first draft of human genetic code was published nearly a decade ago, on June 26 2000. Since then, the cost of sequencing has fallen by a factor of 14,000.