One diagonal, five shapes
A monitor's size is quoted as a diagonal, and a diagonal says nothing about shape. Here's a 27-inch screen in each common ratio:
- 16:9 is 23.5 × 13.2 inches (59.8 × 33.6 cm)
- 16:10 is 22.9 × 14.3 inches (58.2 × 36.3 cm)
- 21:9 is 24.8 × 10.6 inches (63.0 × 27.0 cm)
- 32:9 is 26.0 × 7.3 inches (66.0 × 18.6 cm)
- 4:3 is 21.6 × 16.2 inches (54.9 × 41.1 cm)
Stretch the shape and you lose height. The 21:9 version has 264 square inches (1703 cm²) of screen against 312 square inches (2010 cm²) for 16:9, so a 27-inch ultrawide is a smaller screen than a 27-inch standard monitor. Compare shapes by height or by area, not by diagonal.
Compare by height
Height is the fair yardstick. A 27-inch 16:9 monitor is 13.2 inches (33.6 cm) tall, a 34-inch 21:9 is 13.4 inches (34.0 cm) and a 49-inch 32:9 is 13.3 inches (33.7 cm). They're all about the same height, so each ultrawide is a standard monitor with the sides pulled out. The 34-inch is 33% wider than the 27-inch.
Why 21:9 isn't exactly 21:9
The names are rounded. 21:9 is a marketing term for 64:27, which works out to 2.37:1. A 3440 × 1440 monitor is really 43:18, or 2.39:1. Cinema scope film is 1024:429, or 2.387:1, usually rounded to 2.39:1 or 2.4:1. Plain 21 divided by 9 would give only 2.33.
32:9 has no such wrinkle. It's two 16:9 screens side by side, so it's 100% wider than 16:9 at the same height.
Black bars
Content has a shape of its own, and when it doesn't match the screen you get bars. A 2.39:1 film on a 16:9 TV fills the width but leaves 26% of the screen height empty, split between the top and the bottom. Turn it around and a 16:9 video on a 3440 × 1440 ultrawide fills the height but leaves the same 26% of the width empty.
Neither is a defect. It's the price of showing one shape on another. If you mostly watch widescreen film, an ultrawide shows it without bars. If you mostly watch 16:9 video, a standard monitor does.