What do you mean by High-Index glasses?
Index of Refraction
The higher the index of refraction the thinner the lenses will be.
The top row of the chart below is the Index of refraction.
It starts at 1.5 Index and goes up to 1.74.
CR-39 has an index of 1.498
Polycarbonate has an index of 1.58
Hi-Index comes in 1.67 and 1.74
The next row down is a -10.00 power and as you can see the higher the index the thinner the lenses will be in your frame.
The last row is a -6.00 power.
This chart is recommended to be used as a guide.
There are many factors that determine the edge thickness of a lens, such as PD, lens shape, eye size and bridge size.
Again, the higher the index the thinner the lenses.