From Delta Airlines website:
Quote
Rifles
Delta will accept firearms and shooting equipment packaged as follows:


One gun case containing up to a total of four rifles or shotguns, plus shooting materials, and tools
One gun case containing up to five handguns, one scope, and tools
One bow and quiver of arrows and maintenance kit enclosed in a case or container of sufficient strength to protect the bow and quiver from accidental damage
An excess baggage fee will apply to cases containing more than two rifles.


Doesn't look to me that the size of the case matters, just the number of guns within it.