Why do we make our cars so small?

my uncle collects 100's of classic cars, he picks me up in a different one everyday since my parents work. Tomorrow he is picking me up in his 1951 Hudson Hornet. But when I look at the modern cars compared to the one I'm in, I always wonder why we make them so small. I mean, I am 15yrs old, a boy and I am a car nut, I take pictures of every classic car I see, even if its my uncle driving! But the smallest car he has is the Ford Model A. But why does GM make their car smaller? (GM owns GMC, Buick, Chevrolet, and Cadillac). But why do we make our cars smaller even though America was known for their big cars.
Added (1). I live in the San Francisco Bay, California

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