Tiny homes need to make efficient use of space and that includes the bathrooms. A tiny home bathroom must accommodate a bathroom, a tub and/or shower, and a sink in a really small quantity of space.
An exception to this is when a home may share a kitchen and bathroom sink. In typical sized homes, bathrooms typical about 45 square feet and that is about the smaller end of this scale. 45 square feet, however, is far too large for a bathroom in a tiny home.