In Rocket League you can choose between many different-looking vehicles. But there are actually driving skills differences between these "Battle-Cars" in the game.
Rocket League: Battle-Cars - Differences between the vehicles
Even though you might have a different impression here and there, there are in terms of handling no difference between the Battle-Cars. Speed, handling and braking performance are absolutely identical in each of the 11 selectable vehicles. You should be aware of this, otherwise you could get an unfair advantage when performing competitive mix of racing and football game.
Rocket League: Hitboxes make all the difference
Each Battle-Car in Rocket League has a different size Hitbox. This means that every vehicle has different size bodies more or less hit area. This can ever get meaning in games among professionals, because here every centimeter counts. For example, the Merc has a larger hitbox than the smaller Gizmo. But this is the only difference, which does not fall further into the weight in most normal matches.