The arms of my cactus are growing skinnier and I don’t know how to make it grow and appear like the original arm (lower part of the arm). It’s skinnier and has a lighter green color than the loser part of the arm. Should I water it more? Or give it more sunlight?
It's likely something called Etiolation. This is where the plant is not getting enough light in it's growing stage. If you've ever had a house plant start to lean towards a window, you've experienced this. Unfortunately this type of growth is more permanent with a cactus as it grows so slowly. You'll see this on all those Instagram Influencer's posts...
I'd say try to get him some more light. All is definitely not lost and it looks quite lovely and healthy! Might just add to the wonderfully funky look. Over time though, if it stretches too much and then the next growth cycle has more sufficient light, the new growth will be fatter and could become too heavy for the skinny part.