Weber-Fechner Law
Weber-Fechner Law is the finding that you notice differences less as things get heavier, louder, or brighter.
Think of it this way
Add a pinch of salt to plain water and you taste it right away. Add that same pinch to a pot of salty soup and you won't notice any change. The more salt already in the soup, the bigger the pinch you need before your tongue can tell. Each extra pinch matters less as the base amount grows.
Explaining weber-fechner law by grade level
Pick up a light bag, then a slightly heavier one. You can feel the change. Now pick up two very heavy bags. The same extra weight is harder to feel. Your senses notice small changes best when things start small.