Some people mistakenly think that just because a pet is small it needs less attention and care than a larger pet. Take for example goldfish, they require colder water temperatures than tropical fish as well as more space. The goldfish also have higher water filtration requirements, they poop more.
Ferrets look cute but are very active and have special needs, way beyond what a mouse requires. They have to be handled regularly to keep them from developing the nasty habit of biting their handler.
Cats seem to be the pet that most people think of as low maintenance. Sure they look like they sleep all day but when they have those days that they just have to be busy is when you come home to a major clean-up of the broken or torn apart something that the cat just had to play with for 5 minutes.
Birds are one of the most neglected small pet that I know of. How many times I have seen birds kept in too small of a cage. My uncle use to breed canaries and he had an outdoor avery. It was so large that he could walk into it to tend to the birds. He would often tell me that people put birds in cages that mimic the pet store cages not realizing that the pet store cages are meant for display only and are not the recommended size for the bird when kept at home. Much like keeping your pet puppy in the same size puppy pen that you found it in at the pet shop or the breeder's. You have no doubt seen those wooden birdcages that are commonly used for putting plants in for decoration. Well unfortunately I have also seen them used for live birds. One woman kept her parakeet in one until the bird knawed through the wood and got out.
Just because it is a small pet does not mean it has small needs.