You have a habit of raising interesting questions.

Point 1, in the "for what it's worth" category: The bigger the planet, the stronger the gravity. The stronger the gravity, the more resistance against movement an organism will experience. Gravity beyond a certain point (don't ask me) is probably counter-productive for large organisms.

More nerves, more pain? Well... where are the nerves? Our brains, with at least 100 billion neurons, probably more, feel no pain -- themselves. Our brains interpret pain signals from elsewhere in the body. It does make sense that the larger number of pain nerves that are stimulated (by injury) the worse the pain will be. There are limits, sometimes.

Distraction can enable us to ignore pain (provided it isn't too severe, too long lasting). Other sensations, such as cold, can swamp pain (which is why putting ones slightly burnt hand in cold water reduces pain). Acupuncture, hypnotism, relaxation and meditation techniques can reduce pain.

Severe burns are so painful because many layers of tissue (all with pain receptors) are affected. Shingles is very painful when the varicella-zoster virus is very active inside nerve cells. varicella-zoster virus (chickenpox) can cause shingle pain for years. (The chickenpox vaccine does not cause shingles.)

FYI, there are separate nerves for pressure, itching, and for pain. Itching can also be quite severe.

Finally, pain nerves can stop functioning. Diabetes can cause peripheral neuropathy, where the diabetic no longer feels pain in their extremities (feet, hands) and may not notice injuries that can lead to severe infection.

interesting. I knew a guy working on a miniature blimp (as opposed to a full sized blimp) and one of the things he discovered is that when you shrink the blimp you have to change the blimps shape. He speculated the reason was atomic and sub atomic particles are a certain size so when you decrease the size of the blimp you have to change the shape also (to make the blimp still be effective).

A blimp is a helium filled cigar shaped aircraft.

I wonder if this concept would apply to giant humans or mini humans.

No, pain is a process translated as such by your brain, so the more BRAIN cells you have the more capacity for pain you might have. But its also not that simple, because most brain cells are not engaged in processing electrochemical messages from your nervous system Your individual cells, even your nerve cells also do not 'feel' pain, they might respond to stimulus, you can poke at a nerve removed from your arm and it will respond by sending pain messages along its length, but its not feeling pain itself, its trying to send a message to a brain that can process the data AS pain. neither does your brain... its just the processor of the chemical messages you later perceive as pain.

So what would make you feel more pain? Well a larger R-Complex in your brain would be a start and a larger pain processing canter with a greater capacity for types of pain. There is a difference between a numb pain or throb and a stinging pain or stab... but there could be lots of others, just that we don't have that capacity because we have a limited number and 'types' of end receptors that pre-process pain before sending that to the brain. In some cases nerve receptors will react themselves to pain, but they do that instantly and then later you 'feel' that pain.