If you're buying a collar for a new puppy, it might be tempting to select a collar that would also suit his full-grown size to save yourself some money. However, a collar that your puppy can grow into will be much too big and heavy for him to use now. Not only is he likely to be able to slip out of it but it will also be too bulky for his small frame. Getting used to wearing a collar and walking on a leash is a big step for a young dog, so it's always best to start a puppy with a thin, lightweight collar.