So I'm making a basic car model here for fun, but when I try to add the subdivision surface modifier for smoother body, the side of the car become what it looks like in the screenshot. Is there any solution to make the side look less bulky? Any help is appreciated. :D
Subdivision surface give "predictable" results when you are using quads.
Your car's side face is a ngon, so is not a quad, but instead 4+ faces poligon which is triangulated by the modifier and then smoothed by creating a vertex in the middle of each existing edge.
Depending on what you are exactly looking for, you should make some changes on your model's topology.