All babies are different but, on average, formula-fed babies do tend to be a little chunkier than their breastfed buddies.
Breastfed and formula-fed newborns have similar weight gain in the first few months, but formula-fed babes tend to pack on the ounces a little faster during months four to six. (Head circumference and length don’t seem to be affected.) After six months, breastfed babies tend to stay a little slimmer. Nobody is exactly sure why this happens, but some think it may involve water retention and a different composition of body fat.
(Something to note: Breastfed babies are also less likely to be obese as adults.)