For weaker eyes, the number in the prescription is higher, and the lenses must bend the light more to provide clear vision. To bend light more, stronger minus lenses require thicker edges than weaker minus lenses. It's not unusual for a shortsighted prescription to increase over time, meaning lens edges will grow increasingly thicker.
This is where the new high-index lens materials come in. Chemists have created a variety of new plastic materials that bend light more than the conventional plastic used for eyeglasses. This means less lens material can be used to correct the same amount of shortsightedness.