Is there any technical difference between the different car body types in Rocket League? Based on my experimentation, there does not seem to be a difference but I could not confirm it.
According to the developer, the cars have slightly different hitboxes. Other than that, they have identical stats.
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Hit box can change slightly. Other than that, there is no difference whatsoever.
I'll find an official source if I can, but that's what I've noticed from personal experimentation/gameplay.
Actually, I found a ton of sources (although mostly are unofficial) by simply Googling "rocket league hitboxes". Might be worth a look-through although they just confirm what I say above.
In case you were curious about toppers: they are aesthetic only, performance isn't affected.