Smaller means less mass and less weight. The closer to round the frame eye shape, the more likely that the overall appearance will be comsmetically appealing 2. Glass and plastic lenses are available in varying refractive abilities called index of refraction. The higher the index, the more the material bends light for a given curvature and thickness. Glass is heavier but more dense and resistant to scratching and warpage.
Plastic is lighter and more resistant to breakage, but more readily scratched.
One plastic, polycarbonate, is so impact resistant that it is used as a safety lens. It is also important to remember that all ophthalmic lenses filter ultraviolet light. Other factors influencing thickness and weight are aspheric curve design and center thickness. Aspherics tend to reduce edge thickness. Some materials can be manufactured with less center thickness than others, while maintaining stability of the curvatures.