The taller lens has space below the reading well of the lens. This lets you see and focus on your feet and edges of stair steps. So you dont trip.

With the small “fashionable” lens you see below the frame and cannot focus at foot level because that vision is not corrected. As far as floor level objects are concerned you are better off without any glasses at all.

Try explaining this to the clerk in the optical shop at Walmart or Costco. They might call security to kick you out!