It has more to do with style than how hard someone could hypothetically punch. Every athletic adult male has the power to knock someone out if they put everything into a punch. But look at fights like RDA/Lawler, you'd think hundreds of punches from a 170lb man would knock someone out, but not when they're being thrown high volume at half-power.
Most fighters that are perceived as having pillow fists are reallyjust fighting in a way which allows them to win through pressure/volume rather than loading up for a big KO (and gassing when they fail)