How can I get thicker arms? Not talking about lifting weights?

If you are seeing an increase in belly fat then you might want to reconsider your diet. For example, yes you must eat a lot to gain muscle, but it must be healthy too. Not mcdonalds and pizza. If you are following a healthy diet incorporate compound exercises into your workout routine for a more full body exercise. To answer your question about your arms, doing pull ups, chin ups etc will make a big difference. Do as many as you can and increase the amount steadily. Once you're able to do 15 to 20 easily then add a weight belt.

Protein protein and more protein.

Even if you think you have muscle already, lifting weights will do miracles. Making your arms bigger whether you want them big and toned or just a little thicker. I'm assuming you're a female. Lift weights I promise it helps. It is almost impossible for females to get very bulky if that it was you are afraid of.

It depends on bone size and where your muscle attachments are.

A guy with a 8 inch wrist and a short forearm attachment will look like a bruiser.

More common is a little over six inch wrist and a long forearm tendon.this persons lower arm wil always look thin.

lift weights and eat i guess but not just that but just plain work that you lift a lot of weight in will em bigger but it could be genetic or something.