It is hard for babies who are exclusively breastfed to become overweight. It is another wonder of breastfeeding that babies can’t easily eat too much, because breasts auto-regulate and if babies want to feed more often, they simply get more of the watery, foremilk (rather than the fattier hindmilk). So it quenches their thirst, allows for comfort sucking but won’t up their calorie intake significantly… until they are ready for it. Ain’t Mother Nature a star?!