Frames that are narrower at the top and slightly wider or more detailed at the bottom help balance an inverted triangle face's broad forehead against its narrower jaw.
The Styling Principle
Minimize width at the forehead and temples while building width or structure at the jaw, using volume, texture, or facial hair to bring the lower face into closer proportion with the upper face. Frames that are narrower at the top and slightly wider or more detailed at the bottom help balance an inverted triangle face's broad forehead against its narrower jaw.
Sizing
As with prescription frames, sunglasses width should roughly match your face's widest measurement — for a inverted triangle face that means sizing relative to cheekbones.