Some boxes are genuinely excellent. Strong corners. A proper lid. The exact size that surely must be useful for something. And that is how a perfectly sensible person ends up storing empty containers inside other empty containers.
Name the job before saving the box
“This is for the decorations” is a job. “You never know” is an invitation to keep every box that enters the building. If you have a specific use and a place for it, keep it. If not, it can leave without representing a failure of imagination.
Give your spares a physical limit
One shelf or one larger box can be the entire spare-box department. When it is full, choose between what is there and what just arrived. A physical boundary is easier to remember than a complicated rule about acceptable packaging.
Keep packaging you actually need for an upcoming return until the relevant return requirements are clear. A box with a real near-term purpose is different from the six you have been moving around since spring.
Let useful things be useful somewhere else
If someone you know is moving and wants boxes, offer the suitable ones. Otherwise, follow your local recycling instructions for the material. Remove personal shipping information before passing packaging along.
You are not required to find a beautiful second life for every piece of cardboard. Reuse is useful when it serves your home. When the empty boxes make it harder to reach things you use, the balance has tipped. Keep your favorite practical spares and give the cupboard some breathing room.