Basically when the Comp hammer was first designed it was made to fit all the pistols, so the thinner hammer dimension was used. Because most CZ 75 had the narrower slide slot.
When the Shadows were made they used a larger hammer slot in the slide and hence increased the size of the hammer but the comp hammer was never made wider to suit.
Basically the extra mass was not necessary, (we are talking about very little weight)
The low hammer were specifically designed for use on the Shadow Target, because of the Low Mount Adjustable sight, the "High" hammer could hit the lower portion of the blade of the sight.
So when we needed a new hammer for the Tactical Sport we had the factory "push" the dimension of the hammer a little thicker to give a little more mass for the SA only pistol and to make it a little more aesthetically pleasing.
So effectively you can now choose the hammer to match the rear slot size and height. Depending on what rear sight is on the pistol.