Marinella:
No, not all bottoms are as spank'able as a perfect rounded one, but that has nothing to do with the rest of the body.
This is true, I tend to feel a lot has to do with how much the spine concaves at the base. If it concaves too little the buttocks are not pronounced, which renders them less attractive. High heel shoes enhance this curvature on women, as well as lift the sit spot giving the 'pert' effect.
Back in the 70s my mate's girlfriend had what I called a "teddybear" bottom, which has little spankability. She was an attractive girl generally but with a thickish waist and a flat back which made her look like a teddybear. Her bottom was wide but did not project outwards.