Everybody's baby comes in a different size. Someone's newborn might be someone else's 3 month old. This is the range of garments designed for babies who are still in diapers or those puffy training pants! More room in the seat area, if ya know what we mean. Neck openings are larger to accommodate dressing baby.
These will be slimmer than the infant sizes as toddlers are starting to move more and lose that baby fat, and in the larger sizes, no longer wearing diapers.