The short answer is that it isn't just that they make you louder. They do make you a bit louder, but loudness is a psychoacoustic phenomenon. There is more to it than just that. In the Saturday session I mentioned the principle of acoustic masking. That's a big piece of it.
Think of masking this way. If you were standing in the street wearing a T-shirt and the wind was blowing, you would feel the shirt moving in the wind. However, if you got hit by a car driving by, during that time you were getting hit by the car, I doubt you would notice your shirt blowing in the wind. If something else applies more energy to the thing you are applying energy to (in this case a slice of the audio spectrum), you will get a smaller slice of the overall pie.
The prior video I posted showed how the different areas of the sound spectrum were energized as we used harder mallets that extracted more harmonics from the bars. In this video I show a visualization of why that matters in a mix and how harder mallets help to overcome acoustic masking.