Question

A thin lens has a smaller focal length

Answer 1

A strongly curved lens will have a shorter focal length than a weakly curved lens. While not simple single lenses, you can see this in camera lenses, particularly in the objective lens. Here's a 14mm f/2.8 lens, a short focal length, of course. You can get an idea of how thick that objective lens is.

Answer 2

A thin lens has a smaller focal length