Early printers inherited their proportions from manuscripts, where the margin was a place for the hand to rest and for later readers to leave notes. The text block sat high and toward the spine, with the widest margin at the foot of the page.
Modern paperbacks often squeeze those margins to save paper. The result is a page that feels crowded even when the type is well set. Giving the text room is less about luxury than about rhythm: the eye needs somewhere to turn.