A new study examines a phenomenon that perhaps we have subconsciously noted but never deeply examined: Firstborn women are more likely to be overweight or obese than their younger sisters. It’s published in the Journal of Epidemiology & Community Health, and it’s the largest study to research this trend in women.

Previous studies have shown that birth order is linked to weight and height in men — and researchers wanted to see if this happened among women too. Using data from the Swedish Birth Register, the researchers focused on 13,406 sister pairs — nearly 29,000 participants.

The researchers found that at birth, firstborns were actually much lighter than their second born. But once they had become adults and were three months in pregnancy, their BMI was 2.4 percent higher than their second born sisters. Firstborn women were also 29 percent more likely to be overweight and 40 percent more likely to be obese than second born sisters, according to the results. Not only were they heavier, but firstborn women also tend to be a little taller — a finding that matched with past studies’ results on men.

But perhaps the most interesting take on the results is the notion that decreasing family size in America might be contributing to the obesity epidemic. Families have become smaller over time for many reasons; “consequently, the number of one-child families is steadily increasing,” the authors wrote, “so that, inevitably, there has been a progressive increase in the proportion of first-borns in the world’s population.”

The researchers claim that the smaller the family size, the more likely the children are to be overweight — simply because more firstborns who are only children means more obese people.

“Our study corroborates other large studies on men, as we showed that firstborn women have greater BMI and are more likely to be overweight or obese than their second born sisters,” the authors wrote. “The steady reduction in family size may be a contributing factor to the observed in adult BMI worldwide, not only among men, but also among women.”

Of course, this is quite a giant leap to make assumption-wise, and no solid research has been done yet to solidify this link. As this is an observational study, more research will need to be completed to better understand it.

Firstborns may be fatter, but they also tend to be more successful — academically, financially, and career-wise, according to past research. In a 2014 study, researchers noted that firstborn females were more likely to be ambitious in their lives due to more parental guidance, investment, and nourishing while their later siblings lagged behind.

“It is interesting that we observe a distinct firstborn advantage in education, even though parents in modern society are more likely to be egalitarian in the way they treat their children,” Feifei Bu, an author of that study, told The Guardian. Of course, this isn’t always the rule.

Source: Derraik J, Ahlsson F, Lundgren M, Jonsson B, Cutfield W. First-borns have greater BMI and are more likely to be overweight or obese: a study of sibling pairs among 26 812 Swedish women. Journal of Epidemiology & Community Health. 2015.