People that have strong glasses diopter ( beyond +/-6.00D) are usually advised to choose ultra-thin glasses lens. As a new kind of optical lens, ultra-thin glasses lens have high refractive index (about 1.74), thus turn out to be thinner and lighter than normal lens for the same high diopter.
Resin lens with a reflection index of 1.67 or 1.74 may be suitable for you.
Besides, smaller glasses frame can also bring you thinner lens.
Usually, the high-index lenses are the thinnest and lightest glasses lenses, especially the type of high-index 1.67, which is also suitable for people who has strong prescriptions and who wear bifocals. And you had better choose the smaller and rounder frame to fit the glasses. Otherwise, it would improve the thickness of your glasses finally as well as plastic frames.
It is hard to tell the thinnest eyeglasses lenses. Generally speaking, for the same prescription lenses, the higher the index of the lenses, the thinner the lenses. However, if you have very slight prescription, you needn't buy some high index lenses such as 1.74 index lenses. That because common 1.50 index lenses can fulfill your needs.