It's because after you take part in a lot of exercise, your fat becomes muscle, but at the same time, your fat doesn't completely become muscle. Now your arm is muscle and fat ~

Because the shape of muscle is more linear, and fat is unformed, the combination of the two makes the arm look thicker. . .

So ~ ~ ~ must continue to exercise.

Strive to convert all fat into muscle ~ ~ ~

This way, the arm will be linear and won't look thick ~