Cats of all sizes lounging in boxes of all sizes is more than just an internet phenomenon; it's a representation of how domesticated felines deal with stress and anxiety when there are no caves or tall trees nearby.
Studies show that a comfy box relieves stress for cats, especially in a new place. Cats are ambush predators, and the feeling of being surrounded by a box provides them the comfort of knowing they are safe and able to defend themselves if need be.
The Kessler & Turner Cat-Stress-Score considers ten factors for observation including respiration, posture, and tail to calculate a stress score between one and seven. One study showed that a group of cats exposed to a new place differed greatly in the time their stress was reduced based on whether or not a box was available for sitting. Cats with boxes in a new place had rapidly reduced stress compared to cats without boxes.