It's simple.
Haki is like steel. Or at least it makes things harder.
If you add Haki to a stick, the stick becomes as hard as steel.
If you add Haki to concrete, the concrete becomes even harder than a stick with Haki.
If Haki = 10 and your strenth = 10, your overall output would be 20.
That's why Luffy was able to defeat the Gorgon sisters without Haki even though they used Haki.
Initially, Luffy's attacks got deflected by their Haki defence because his attacks were too weak.
As soon as Luffy used Gear 2nd, his speed and power increased, thus his speed and power was greater than their combined defence level and Haki defence combined.
He defeated them with brute strength enough.
Another example is Luffy vs Katakuri.
Katakuri's Haki was so hard that Luffy's fist hurt from punching him.
However, Gear 4th gave Luffy increased physical power while his Haki strength remained the same. The strength of an attack is a combination of both Haki and physical attributes. Luffy was able to overcome Katakuri because Gear 4th just has immense power especially as it's combined with Haki.
So, yes, the harder or stronger someone is without Haki, the more effective Haki will make that attack.
That's why a stick imbued with Haki acts like a steel pipe instead of a stick because it's made immensely harder due to Haki.
Basically, if something is harder, it will do more damage and it will be more protective. That's literally all Haki does fundamentally. Just makes things harder.
You'd feel more pain being hit with a steel pipe than with a twig even if you got hit by the same amount of force.