Actually, the passenger windows in commercial airliners used to be much bigger.
Here is the interior of a Ford Trimotor from the late 1920s:
The Douglas DC-3 of the late 1930s also had relatively large passenger windows:
By the late 1940s, larger piston-engined propeller-driven airliners had pressurized cabins, which allowed them to fly at higher altitudes, above most of the turbulence and clouds, for greater passenger comfort, while compressing the air in the cabin to simulate lower-altitude air density — also for passenger comfort. Here is the interior of a DC-6 from that era; notice that the windows of the pressurized cabin are somewhat smaller than those of the earlier DC-3 despite being penned by the same designer, and now have rounded corners:
The early 1950s saw the emergence of jet airliners, beginning with the DeHavilland Comet Mk. I, which, despite having a pressurized fuselage/cabin that flew at an even higher altitude with an even greater pressure differential than the earlier, pressurized propeller planes, reverted to larger, square windows:
This turned out to be a tragic mistake on the part of the Comet’s designers. The square-cornered windows became focal points of stress in the metal fuselage structure, especially with repeated cycles of pressurization and de-pressurization, which resulted in rapid metal fatigue, much like when you bend a paper clip back and forth until it snaps. That’s exactly what happened — not just one, but a few Comets snapped apart in mid-air, crashing. When crash investigators finally figured out the cause, the Comet was re-designed to have smaller windows with rounded corners, and went on to have a successful career. All subsequent jetliners have followed that formula, for the reasons indicated. Here is a later version, the Comet IV, showing its smaller, rounded windows: