The lead author said the difference may come down to how much of a role babies play in deciding when to stop feeding, instead of mothers or fathers forcing them to finish a bottle.
Li told Reuters Health some researchers believe that "if the babies are fed with the bottle, they will gradually lose their self-regulation of their energy intake and the internal cues of satiety and hunger."
To look at the link between weight gain and feeding, Li and her fellow researchers followed about 1,900 babies from across the U.S. born in the mid-2000s.
Through a series of surveys sent to their mothers, the researchers asked for — among other things — babies' weights at different ages and how often women breastfed, pumped their breast milk or used formula.
Babies who were only fed from a bottle — either with only breast milk or only formula — gained about three ounces more per month compared to those who were solely breastfed.
After that, the findings got a bit complicated.
When moms did a combination of breastfeeding and bottle feeding with human milk only, babies didn't gain any extra weight — but if they both breastfed and bottle-fed with formula, their babies gained an additional two ounces each month, on average.
And when moms exclusively bottle-fed, but alternated between using human milk and formula, their babies grew similarly to solely-breastfed babies.
It's not totally clear why babies fed a combination of bottled breast milk and formula may not have gained additional weight, the researchers wrote in the Archives of Pediatrics & Adolescent Medicine.
But, "The key message out of this study is that breastfeeding really is the first feeding choice for the babies," said Li, who added that supplementing breastfeeding with breast milk from a bottle is a good second option.
The new study is only a piece of the growing evidence that breastfeeding appears to be the best choice for a newborn, and may protect against obesity later in life.
In the same journal, Tessa Crume and her colleagues published findings suggesting breastfeeding for at least six months may not only protect kids against being overweight later in childhood, but also against being underweight.
Simply put, overweight kids might have been even more overweight if they weren't breastfed as babies, the researchers reported. At the opposite end of the weight spectrum, there was a suggestion that underweight kids might have weighed even less if their moms hadn't breastfed them.
Those findings are based on weight and body fat measurements, as well as breastfeeding history, for 442 children between the ages of six and 13 years old.
"It is suggesting that breastfeeding has a growth-regulating effect — preventing extremes," said Crume, from the Colorado School of Public Health at the University of Colorado in Denver.
IN AN IDEAL WORLD
Despite growing research in favor of breastfeeding, the authors of a linked commentary say the new findings may put too much pressure on new mothers.
"Pediatricians should deliver their expert advice with empathy, being mindful of the gap that always exists for parents between doing what is ideal for their children and doing what is possible," according to the editorial.
Co-author Dr. Jeffrey Wright, a pediatrician from the University of Washington School of Medicine in Seattle, told Reuters Health there could be anatomical or economic reasons why mothers cannot breastfeed their children.
Some mothers have to return to work, fathers may want to help with feeding and the parents may want to know how much milk their baby is getting. Wright said all of these reasons may keep mothers from breastfeeding.
The American Academy of Pediatrics recommends mothers exclusively breastfeed for six months, and continue breastfeeding as foods are introduced until at least 12 months.
"There were millions of babies raised on formula well before the obesity epidemic started," said Wright. "Each family should weigh the benefits they see against the hassles they take to get there, and the father should be involved with that discussion."
As with any research, the new studies also have limitations — including that neither can prove breastfeeding directly protects kids from gaining too much weight.
But researchers agreed that employers need to do what they can to promote breastfeeding, such as having onsite child care and allowing new parents to work part-time.
"Breastfeeding is really the best feeding for the babies, and needs to be the first choice," said Li.
If that's not possible, she said it's important to pay attention to the signals a baby sends out to prevent overfeeding with a bottle, such as keeping their mouth shut or not wanting to suckle.