It doesn’t need to be. I had shared bathrooms in college dorms and in a European facility for unmarried adult asylum seekers and in various hostels and they were absolutely fit for purpose.
The bathrooms at my workplace are fit for purpose too, but that doesn't stop people with bad hygiene from leaving a line of fecal matter at the rear of the seat (where the cheeks meet) and not wiping away sweat, lint, and hairs, or prevent them from missing and peeing on the floor.
I feel like a toilet, shower, and sink could be added to an efficiency apartment for 3m².
If people want to see how small a room could get, see ships, especially crew quarters. I could probably design something with a kitchen, bathroom, washer and dryer, bed, desk, and storage for 15m².
