Question about car textures

I´m a newbie to all this staff, so maybe my (stupid) question is: what´s the difference between the original cars and custom cars in terms of textures? GP4 performs mostly better with the original cars (lower PO). My thinking was about the hi res textures, but the original carset has them too, so how can it be such difference?

original hi res are 512x512 pixels. and i think there is 2 per team.
newer hi res have 1024x1024 and for some teams theres 4 of them. so 8 times moe resolution?!

____
Tony

Thanks for your answer