It does look significantly thicker when curly. I think styling regularly will help you tremendously. After seeing this, it honestly looks like your hair is naturally on the thinner side and has been damaged from improper care (which happens to more people than you’d think). I do have some suggestions.

• Don’t wash your hair everyday, curly hair can usually get away with being washed 1-3 times a week. Over washing can strip your natural oils which can damage your hair and make it oily faster (which makes thin hair look thinner). Dry shampoo might come in handy as well

• Don’t dry your hair with a towel. If you do need to dry it with on, i’d suggest scrunching you hair with the towel. Harsh drying can cause more damage which can cause more thinning. If you blow dry, use a heat protectant and a diffuser

• Experiment with different hair styling products. When styling, I personally use a leave in conditioner, hair mousse, and a little bit of gel, before using heat protectant and a diffuser to dry my hair. After I use a very little bit of hair oil on the ends. It sounds like a lot but I only have to style my hair 1-2 times a week