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This predominantly depends on the strength of your optical. The higher the optical, the higher index is required for ensuring your lenses are as light and thin as possible. 

Here is a picture for reference:

A specific example is shown below on the indexing of a -4.00-lens optical on how thin the lens could be.

Each lens type is made from different materials. The higher index materials allow for flatter curved lenses that are still able to maintain an aspheric design. Both of these features help produce thinner and lighter lenses.

Please note: The ultimate thickness of the lenses is determined by the frame size (including lens height, lens width), frame material and your PD. If you choose a big frame the lens edge will undoubtedly be thick. Frames with thin metal rim will also make the lens look thicker. If the frame is not the best for your PD, the lens may not be beautiful and thin as you expect. 

Your PD may be written in three ways:
1. PD (OU), written as "64", means the Binocular PD which is for both eyes. 

2. DUAL PD, or monocular PD, consists of two numbers and is the distance between the centers of each pupil to the bridge of the nose
Dual PD is written as two numbers, for example, "32/32", the first one "32" represents the PD for the right eye, and the second one "32" represents the PD for the left eye. 

3. Sometimes PD is written as "62/59" or they are labeled "Far" and "Near". Your PD is usually measured for distance vision, which is 'Far PD", or "62" in this example. For reading glasses, doctors measure your "Near PD" or "59" in the example.
Always enter your "Far PD" for distance vision eyeglasses and enter your 'Near PD" for your reading glasses only. For most people, the difference between Far PD and Near PD is about 2-3mm.