It doesn’t matter if it’s big or small, cats don’t seem to discriminate when it comes to a good cardboard box.

It is instinctive. Cats are king of the jungle and they like to stalk their prey. A secure hiding spot is attractive and a cardboard box gives them a vantage point from which to watch the world around them (and have the occasional swat at an ankle that passes by).
It makes them feel safe. Inside a cardboard box they can only be approached from one side, so there is no possible surprise attack from behind.
It is a great spot for a snuggly sleep. Considering cats sleep something like 18 hours a day, I’ll trust that a cardboard box is the best spot they have found.
Boxes are tons of fun. You only have to watch your cat with a box to see just how awesome a cardboard box is. They can jump around, pounce on things, climb on it, or even chew on it. Sounds a bit like the sort of fun we had with boxes as kids, so we totally get it.