Reflectance
How much light a surface bounces back rather than absorbs. High-reflectance pale surfaces spread daylight deeper into a room and make walls feel further away — extending the perceived size of a space at almost no cost.
Cheapest way to feel bigger
Repainting a dim room from a deep colour to off-white can make it read noticeably larger, because the light now reaches the back wall instead of being swallowed.