If it is like everything in the industrial and tractor world the head gaskets are always made to fit the largest engine in the "family" produced, that way they only have to make one head gasket and it will fit all the engines from small to large bore (within a range). For example an M Farmall has a 3 7/8", Super M had 4", Late 400 and 450 had 4 1/8" Bore, all these engines use the large headgasket that was made for the 450 4 1/8" bore.
The same for the 6.9 and 7.3l they just use the larger gaskets on both the 6.9l and the 7.3l because there isn't that much of a difference. Now if you put the smaller diameter gasket in the larger bore engine you would get hot spots and have problems but you don't really find the older small bore gaskets anymore. It is cheaper for them to just make one gasket to fit all the bores.
Oh and as a side note, on the older gas engines, you can custom make a headgasket (usually copper) that is only as big as the bore to increase compression and gain a small amount of power, or you can just do what I do and just bore the engine to size of the headgasket and get both the benefits of more displacement and more compression, because bigger bore = more compression when everything else is constant.
