Having a glass shower door over a curtain opens up a tiny shower and makes it appear larger. Having a curtain is like having another wall, and it closes the space.
Backlighting your bathroom mirror amplifies the light in the room and makes it feel bigger and brighter. This hack works great if your bathroom doesn’t have room for vanity lighting.
Having light and bright colors like baby blue or mint green helps reflect the light coming in from the window if there is one.
High gloss paint will work to amplify the natural light coming in from a window by acting as a reflector.