Here's the new Vauxhall Corsa. Squint at its outline and you'll see the ghost of the old one, but all the panels are new, and so is the interior, most of the engines, most of the suspension, and the whole structure forward of the windscreen. So they're being fair in calling it a new car.
Does it carry the proportions and glazing of the old one for any reason other than expediency? Well Vauxhall claims that Corsa buyers, of whom there are lots, actually very much liked the size of the old car as well as its basic look. The supermini class has stopped growing, and the Corsa too retains its 4m length. People want compactness. The old car was well-packaged, so again the new one doesn't mess with the cabin space.