Don't discount the weight of 2 extra pieces of glass. Glass it a lot heavier per unit area than sheet metal. The late AMC Pacer demonstrated that by being a lot too heavy for its size.
Then too are the 'appointmenst.' My Suburban with the extra acre or two of glass and extra seats is a lot heavier than its pickup truck parents. It sits empty at the curb displacing 5,800 pounds. Yes, underway it also handles like the 3 tons of rolling stock that it is, but boy-oh-boy will it carry passengers and freight.