Seven-year-old girls who are underweight are also running the risk of developing breast cancer, it has been found.
Conversely girls who are slightly overweight at a young age are less likely to develop particularly aggressive types of tumours which are very difficult to treat, say scientists at Karolinska Institute in Stockholm.
The research, published in the Breast Cancer Research journal, could pave the way for old childhood photos being used as a means of estimating a woman's risk of breast cancer.
Though a role of childhood body size in postmenopausal breast cancer risk has been established, less is known about its influence on tumour characteristics, the researchers said and revealed, "We studied the relationships between childhood body size and tumour characteristics in a Swedish population-based case-control study consisting of 2,818 breast cancer cases and 3,111 controls."
The population was split into three groups depending on whether they were 'lean,' 'medium' or 'large' build when they were seven years old.
The researchers used photographs and their own memories as a basis.
Surprisingly they found that women who were bigger when younger were less likely to develop the disease in the menopause.
Previous research has found that obese females are much more prone to breast cancer. They are also 50 per cent more likely to die from the disease.
They concluded, "Greater body size at age 7 is associated with a decreased risk of postmenopausal breast cancer, and the associated protective effect is stronger for the ER-negative breast cancer subtype than for the ER-positive subtype."
They say their findings could have important implications in determining a woman's risk, but they do not know why skinny girls are more likely to develop breast cancer.
Jingmei Li, who lead the research, said: "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 otherwise elevate breast cancer risk.
"There remain unanswered questions on mechanisms driving this protective effect."
She added: "Given the strength of the associations, and the ease of retrieval of information on childhood shape from old photographs, childhood body size is potentially useful for building breast cancer risk or prognosis models."
The study also showed that larger girls were less likely to develop what are known as 'oestrogen receptor negative' tumours, one of the most deadly forms of the disease.
Breast cancer is the most common form of cancer in women and up to 1 in 9 will get the disease at some point in their lives.