You shouldn't bottle feed. If mom has milk, use her to feed the baby (it'll have a much greater chance of survival without changing milk). You can do this by placing her in a box with some hay and petting her head to keep her calm as you place the baby under her and let it nurse (make sure it doesn't get cold and that she doesn't stomp on it).
How much smaller is it, and does it look any different otherwise? Peanuts are common in Netherland Dwarfs. They are the ultimate runts, resulting from getting two dwarfing genes, and they inevitably die (though there are rare cases of them surviving).