That is very interesting, as we have just updated our burger menu to a preferred thickness.
Of course, our website is running on Squarespace 7.1, so in case you are on the same, try using this css:
/*burger menu thickness*/
.top-bun, .patty, .bottom-bun {
height: 3px !important;
}
/*end burgre menu thickness*/
p.s. you can adjust the thickness to your liking by changing value next to px (currently set to 3px)
hope this helps!