User blog comment:Rupert Giles/Layout Changes: Breakpoints and Typography/@comment-4189499-20150520014724/@comment-4189499-20150520025940

See? Pixel width is high, browser is 100%, but as the physical size of my screen is small with just a high pixel density, the width looks completely normal, as in almost identical to what it looked like yesterday. So the widths can't be based on pixel count alone.