One common reason people choose smaller mammals as a pet is if they are restricted with the amount of space they have for a pet. A prime example would be something living in a small apartment in the inner city. Chances are that there would not be enough room in the apartment for a dog to be comfortable and have room to play. On top of that, it can be a real hassle walking a dog up and down stairs to take them outside and some dogs will require training to be able to walk in the city without problems. Training can be costly and the entire experience of having a dog in a small apartment can be full of hassles so in this situation it is probably better to go with a smaller pet.