The sidewall height (aspect ratio, I believe, in tire industry terms) increases with tread width.
A 60 series tire has a sidewall height approximately 60% of the tread width.
A 50 series is 50% & so on.
Wider tread = taller sidewall.
I'm running 285/75 R 16" & keep em' at about 45 psi in summer for a little less rolling resistance & will be dropping them down to about 35-38 psi for a little better traction this winter.