The box on the left (and top) is a good conduct medal in a box that is larger than the Victory Medal box on the right (and bottom). I have always wondered if the larger boxed service medals you encounter from WW2 are naval issue, and the more slender boxes are for army? Or is just a matter of manufacturer? Sorry if this is a dumb question, but the varying box sizes has always left me perplexed.

Any boxes that refer to the Philadelphia QM Depot are Army. ( PQD , Phila QM Depot , ETC ) 

As pointed out, the label on the box will usually provide the information needed to determine if it is an Army or Navy contract. The boxes can vary from one supplier to another, and even differ from one contract to another by the same supplier.

I have noticed that Army Good Conduct Medals are larger than the boxes for the theater service medals.

Thanks guys