A good boxing punch can hit every bit as hard as a good karate strike, I agree. It could have just been the training I got, but boxing just seemed to use a different set of muscles than karate to strike with. Plus, I was basically a cripple at the time, so I may have been overcompensating with my upper body, since I couldn't engage my hips/knees.
I would basically stand with a squared stance, with my left leg slightly forward, and get reach with my punches by twisting my torso. My arms themselves would stay loose, and very quick, but the twisting motion would wear me out, especially if I was getting hit in the torso.
In karate, now I move my lead leg a little farther forward, and get reach by twisting from the hips, so that my shoulders and hips stay together. It's just as fast, but uses a lot less energy, since I'm using my larger muscles to move, and my smaller muscles to stabilize.
The other energy-saver I found was in the hook vs. "hooking strikes". In boxing they were delivered by a combination of throwing the hand in a circular strike, combined with a twist of the torso to transfer the power. In karate I was taught to throw the hook as a straight technique just to the side of my target, and when I hit the end of my reach, let the strike "snap" to the inside, like the tip of a whip. It delivered less raw weight, but a lot higher velocity, which mathematically works out the same as far as kinetic energy transferred. But it uses a lot less energy for me.
I guess boxing taught me to think of a fight in terms of momentum (mass x velocity), where karate taught me to think of it in terms of kinetic energy (1/2 mass x velocity^2). In other words: "club-head speed".
But I still jab with my shoulder rolled over for protection. It's a marvelous defense. You can still focus the hit on your two "rams head" knuckles, you just have to pull the "pinky side" of your fist back towards the elbow. It protects your weaker knuckles quite nicely.