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Good replies already, but I’d like to look at it from an energy perspective, since the volume of sound from your guitar is ultimately a function of how much energy goes into it. So, let’s get physical. 😉
We know a lighter string takes less energy to excite than a heavier string, but because it is lighter and pulls with less tension, it also puts less energy into the guitar than a heavier string vibrating at the same amplitude. But, since the heavier string is heavier and pulls with more tension, the heavy string needs more energy to get that same amplitude. In other words, light string easy but weak, heavy string hard but strong. So far so good?
Now that energy is, of course, coming from your finger, and energy is a function of mass and velocity. So, the faster the finger moves, the more energy goes into the string. (Let’s assume the mass of your finger is pretty much always the same. 😁)
Your finger also has harder bits and softer bits, and the hard bits will transfer more energy than the soft bits.
We’re definitely taking the long way home here, but all of this is why a good hammer-on is FAST. Snap that finger down, and use that well-earned callous to really slam the thing. Then keep it down — we don’t want to damp those vibrations we just worked so hard to make.
For practice and strength-building, I like to pluck an open string with moderate force, then hammer on after. The hammered note should be about the same volume as the plucked one, and if it isn’t yet, just keep at it, and eventually it will be. 🙂