I took time to review your profile, but, unfortunately, you haven't yet taken time to complete it. I also took time to download, and rotate the one shirtless picture that you have online. Because you didn't complete your profile, I'm guessing, but, you look like you're on the verge of starvation in the shirtless picture.
First things, first. If you haven't, you need to have a blood workup done. Have them do a CMP (Complete Metabolic Panel), along with testosterone, and free thyroid. As well have your fasting blood glucose tested. Sometimes, folks as thin as you have diabetes and don't realize it. Sometimes, guys as thin as you have thyroid disorders, and don't realize it. Sometimes, guys as thin as you have eating disorders and need professional help. Sometimes, guys have never eaten nor been active, which is easily fixable.
The most important thing you can do for yourself is to eat. Period. No discussion. That's the honest truth.
I'd like you to download the USDA SR21 food calculator, and track your calorie intake. If it's below around 3000, you probably aren't eating enough.
You didn't take time to list your activities, nor weight, so it's hard to speculate on how many calories you require on any give day.
While you say you don't want to be big / muscular, you still need to eat. If you're eating and still so thin, then something is wrong. You're either not eating enough, or you've got some underlying issue.
Lean muscle mass burns calorie at rest, so, it's easier to stay lean if you do resistance training and acquire muscle. Only muscle increase your metabolism as you grow older.
Doing lots of cardio is exactly what you DO NOT want to be doing now. I'm afraid that's incredibly bad advice. You need to eat, lift, and find out if you have some other issue that's keeping you so underweight.
As thin as you are, doing lots of cardio could actually damage your organs if you continue to get thinner. It's real bad advice. You need to eat. You need to lift. You need to gain about thirty pounds, while doing some occasional cardio, but just enough to maintain your cardiac threshold, or improve it.