why is it... so big? if I wanted Huge Mice I would get a rat... the appeal of mice is largely in their lack of largeness.

It's because black tan is a show mouse breed. Show mice are typically twice the size of a regular mouse and usually have absolutely stellar temperaments. Also the appeal of fancy mice may be their lack of largeness but the appeal of show mice is that they have super thick, incredibly soft fur and they like to be pet.

Also "tan" is the type of mouse, and refers to the gene that creates the belly pattern. It's a gene like fox (fox causes a white belly, tan causes a tan belly), and it comes in other colors like blue tan and dove tan and chocolate tan.

Well. I’m obsessed.

Even the biggest show mice are much smaller than the average rat. I haven’t seen a dwarf rat in person but I believe even they are quite a bit bigger than a show mouse. There are so many fun varieties of mice, breeding them was the most enjoyable experience I’ve had in a long time. Even though they can be shy, and their lack of largeness is appealing in its own right, I would still love them if they were rat sized or larger.

Some of my show mice:

Marble, a black merle piebald, dressed up for my family’s annual holiday card

Satin siamese splash fox pup, showing off her white belly

Peaches, yellow merle, perched atop the bottom of a wine glass

Homozygous albino, versus heterozygous albino and siamese splash (himalayan splash), versus heterozygous albino and beige (ivory)

Siamese with the fuzzy coat type (not to be confused with the “fuzzy” feeder mouse and rat size), named Knuttsach Jones