Hello!
Thickness is greater for the bass strings than for the treble strings of a guitar because the mass of the string influences the note that you hear.
When strings vibrate, they do so at a certain frequency. Depending on the frequency the note that we hear will be different. Strings with higher masses will vibrate at lower frequencies, producing a deep bass sound. String with lower masses will vibrate at higher frequencies, producing a high pitched sound.
Have a nice day!