Sometimes less pad is even better depending on the purpose and cut of the chamois/shorts. I have some tri-shorts that have a small thin pad and they're more 'comfortable' than some others I have with larger pads. As a mfg of bicycle shorts, we've been through dozens and dozens of pad thicknesses, materials, shapes and designs...it's an ever evolving (well, changing - if not evolving) piece of the puzzle.
As for why not on the seat? It's not just the pad on bicycle shorts, but the lack of seams and bunching material that tend to help the comfort aspect. Putting more pad on the seat would not fix either of these issues.