Ho to set default post width smaller

Hello,

I find the default post width too large when I’m full screen (1280) on a PC (left margin is something like 46 px)

If I resize my browser window to make it a bit smaller (like 1170), the layout adapt (left margin is now like 85 px).

I would like to make this second layout the default layout, is there a way to do this ?

Thank you !