David
I have never had an engine or tender in kit form, so I don't know how big their boxes would be. . I do have an unbuilt boxcar kit for comparison. Looking at how they are assembled, I would guess a tender box would be a bit bigger than the boxcar. If I were to guess, they bought an off the shelf carton for the sets, then packed the kit boxes in with a bit of filler of some sort to fill the empty space.
Steve