optical sizing

[luis jure]

hello list,

in the essay, under "Optical sizing", we can read:

"At smaller sizes, LilyPond uses proportionally heavier lines so the music
will still read well."

is there a way to control this globally? 

i'm using #(set-global-staff-size 13), and i find this compensation a
little bit excessive to my taste. i would really like all the lines a tad
bit "less heavier".

thanks,

lj