The photo to the right illustrates this... A "small" Ferrari 308 that is in much larger scale - but almost the same physical size - as an enormous Fire Truck.
As a kid, the scale issue scarcely bothered me. As an adult, it irritates me immensely. Most "1:64" scale commercial trucks are much closer to 1:90 scale, and you need to buy trucks from the larger "super-king" ranges in order to get trucks that are really 1:64. Even within ordinary passenger cars you see the scale issue - with Matchbox' compact early '70s Ford Capri dwarfing the full size late '70s Mercury Cougar Villager Wagon. Sometimes I try to get around the issue by buying small cars (Honda Fits,etc.) from Tomica, who tends to run slightly smaller than true 1:64 scale, and larger cars and trucks from Siku, who often runs closer to 1:55 scale.
Over the years I've tried to train my mind to see true scale, closely looking at 18 wheelers and cement mixers on the road, and comparing how much larger they are than ordinary cars. So I was amused last week when I saw these 2 1:1 scale red 2 seat convertibles, and thought about how they appeared to be different scales. The tiny back-end of the Triumph Spitfire is dwarfed by the fat, high rump of the Cadillac Allante. The Cadillac appears to be 2X wider than the Triump. And yet, they are both accurately sized 1:1 cars...