No matter whether someone exercises or not, the muscles are fixed. When we say xx muscle, we mean that the muscle is thicker and protrudes from the reference plane of the body.

Because your thin datum is low, your muscles stand out on their own.

There are two possibilities for how to eat fat. One is that you do eat a lot (more than 2500 kilocalories a day), but there are still no extra calories saved. This may be related to your digestive function. I suggest going to the hospital.

The second is that you think you have eaten a lot and are full, but you don't actually consume a lot of calories, so you won't get fat.

If you want to gain weight, you'd better count calories every day and eat something with high calories. Try to eat vegetables and fruits every day to get fat.