That's the conclusion of this study from Australia:
After the researchers adjusted for factors including socioeconomic status and years of education, they found that women who were overweight or obese were less apt to attempt breastfeeding at all and those that did were less likely to continue breastfeeding. ...
Overall, the researchers found, overweight or obese women were 76 percent more likely to stop breastfeeding before their infants were six months old than their normal weight peers.
A number of factors could help explain the findings, the investigators say. For example, excess weight may change a woman's hormonal profile, making sustained lactation more difficult, or it may be harder for an infant to "latch on" to breast tissue if the mother is overweight or obese.
The researchers also report that overweight and obese women were more likely to have pregnancy complications and C-sections than normal-weight women.
Here's one factor they didn't talk about: Lactation consultants and nurses who don't know what to do with big boobs.
When I had Josie, my already-generous breasts swelled 3 cup sizes (and stayed there--I'm still dealing with them). The two lactation consultants sent to my room to help me had NO idea what to do. They were both B cup gals, and very obviously and visibly uncomfortable when confronted with a living Venus of Willendorf. We left the hospital still unable to breastfeed. If I hadn't found another large-breasted mom through La Leche League--and if my mom hadn't helped me pore through every line of every baby book we had checking off obstacles--I would have given up and gone to formula. As it was, Josie and I were a happy breastfeeding pair for three years.
My experience with Louisa was different. The lactation consultant I had was also large breasted (though more normal sized elsewhere), I'd had some experience, and Lou was/is a natural chowhound. 
But it helped to have an LC who wasn't scared to touch me, didn't act vaguely nauseated by my size, and had navigated a good-sized tit of her own with a baby.
Watching smaller breasted women do it, I was always glad I *was* so generously gifted; I had far more positioning options than smaller women. These things were more like hoses. I could point them in all kinds of directions. 
What I'm saying is, body hatred of fat women, by the women themselves and the medical establishment, factors into this. It's not all hormones and/or difficult births.
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