The Whistle GO comes in two variations, but there is no signification size or weight difference between them. It is designed for any pet over 8 lbs. and works great for smaller dogs and cats too!  

Unlike other competitors (in which the product is the collar itself), Whistle GO was designed specifically so that you can keep using your own collar. :)

Whistle GO comes with an attachment piece to secure the device directly onto your pet’s existing collar or harness (up to 1" wide). The device can easily be removed from the attachment when it needs to be charged.