After posting the deboxing video earlier this week, discussion turned to just why the heck boxes are so often far larger than their contents. Each publisher has their own reasons and their own financial pressures to make every one of their decisions, but here are some common reasons.
Standardization: It's just plain cheaper to pick from a whole line of readymade boxes of a uniform size rather than make a custom box for each game. Plus, a lot of collectors like uniformity in their shelf display, hence concepts like the Bookshelf series.
Expansion: When a publisher signs a game that they think has a strong potential for long-term sales, it makes sense to plan for expansions ahead of time. (Usually this applies to card games specifically, like deckbuilders.) The extra space in the core box is meant to be filled with those future expansions.
Transport: Having a fair bit of air in the game box (and the shipping box surrounding the game box) helps keep the contents safe and undamaged. I don't necessarily buy this rationale myself, but I've heard enough from industry experts that I'll defer to their expertise.
Marketing: I can't find any market research supporting this, but I hear from retailers that big boxes that face out to the store floor at eye level really help the game sell itself. The bigger the face, the more it catches the eye.
Pricing: Board games are not a high-margin business and it's tough to just get a product to sell its first print run. Despite gripes from BGGers about "buying boxes of air," the box size communicates its price point. The smaller the box, the more it seems like a cheap impulse buy. The bigger the box, the more it seems worth spending an extra $10.
Loss Control: Smaller items are easier for a shoplifter to pocket, so some boxes get artificially inflated to make that more awkward to manage. Anything smaller has to go up front by the register or within eyesight of the clerk. Again, this demotes those products to the impulse buy category. I hear this is less of an issue with some publishers and stores, but it's still a concern in any retail environment.
This may be a good time to mention that all of Smart Play Games have optional deckboxes. If you don't want another box cluttering up your shelf, keep things minimal and just order a card deck on its own. Browse the catalog here!