The three variables in the attack string are the Surface Area, the Penetration Depth, and the Velocity Multiplier.
For Blunt weapons, it is generally said that the Penetration Depth doesn't have an effect, although i don't know that i've seen the definitive experiment or statement from Toady that proves this yet, but let's assume its true. So it's really the other two that matter.
Surface area, for blunt weapons basically the smaller the better, because the calculation that comes down to applied force per area. So a hammer has a tiny contact area (10), this is probably why it's one of the most effective blunt weapons in vanilla. Mauls and flails have larger contact area (100-200), this is why they're not as regularly effective. Anything over 200 will start to be weak; anything over a 1000 or so is unlikely to do much damage at all (so i suspect your slap and bash don't do too much, but then, it's a spanking, so that's about right?)
Of course, maybe if you have a crazy powerful creature swing it, then the larger weapons can break two bones instead of one, who knows. But for non-superhuman wielders smaller contact is better
Velocity multiplier models how hard you are swinging the weapon, a number in the range of 1000-1250 is for a stab or thrust; 2000 is for a powerful swing; 3000-5000 is a swing from a long shafted weapon like a halberd. Maybe you go higher for a special attack or a magic weapon or something, but that is the normal range.