Onesies are meant to be comfortable and convenient. This means that they’re not meant to be worn too tight or fitting. However, they’re also not supposed to be big and baggy, especially because of the risks mentioned above when sleeping. The best thing to do is find a onesie that’s a perfect fit, somewhere in between tight and loose. It’s better to have to err on the side of looser though, as your baby can always grow into it.