The main reason is that the 787 has a composite construction instead of aluminum. This is more resistant to fatigue, and therefore allows larger windows to be installed. Fatigue is the main reason windows have been kept smaller in the past.

The Boeing 787 Dreamliner features windows that are significantly larger than those on traditional aircraft—about 65% larger than a standard window—primarily due to its advanced carbon-fiber composite fuselage. Traditional aluminum fuselages are prone to fatigue and stress concentration around openings; therefore, windows must be kept small and oval-minded to maintain structural integrity. However, the 787’s composite material is much stronger and more resistant to fatigue, allowing engineers to cut larger holes without compromising the airframe's safety. Beyond the structural benefit, the larger windows were a strategic design choice to improve passenger experience, allowing even middle-seat travelers to see the horizon. These windows also utilize electrochromic dimming technology instead of plastic shades, allowing passengers to darken the glass while still maintaining a view. This design reduces cabin claustrophobia and creates a more open, airy atmosphere that has become a hallmark of the Dreamliner’s interior.