Generally, the Kitchen Gallery will specify with the countertop fabricator when a customer would like a backsplash that it be milled down to 3/4 inch thickness. Alternatively, a backsplash can be left at the countertop thickness of 1-1/4 inches.
Keeping a backsplash at 1-1/4 inch thickness may be necessary because the slab material is fragile and may break during the milling process. In most cases, this will only apply to granite or marble slabs. Man made quartz is not fragile and can, in most cases, be milled down to the 3/4 inch thickness.
It is the Kitchen Galley's preference to go with a 3/4 inch milled down backsplash. It is our opinion that the 1-1/4 inch thick backsplash is "too chunky" looking. Unless the homeowner would like to do a tile backsplash on top of the granite or quartz 4 inch backsplash. The thickness of the tile (after it is installed) removes the "chunky" look.
Below is a video further describing the above.