Choosing between Echo Large and Echo Small depends mainly on the size of your dog. The operating principle remains constant, and the signals emitted by Echo-Large and Echo-Small remain the same. The differences between the two cases include: Size: small or large Battery life: Approximately 90-120 days on Echo-Large and 40 – 60 days on Echo-Small. Double sound emitters on Echo-Large and single sound emitter on Echo-Small. The detection angle: more open on Echo Large If your dog is a medium size, we suggest opting for the large enclosure. This will optimize battery recharges and benefit from the two sound transmitters, thus preventing your dog from disturbing the sound field when his nose is on the ground.