In a box and whisker plot, why are some boxes different sizes than others?

The left edge of the box is the 25th percentile, the right edge is the 75th percentile. The width of the box is the difference, called the interquartile range.

Some data sets will have more widely spread out data than others, or differ in all sorts of ways that will give rise to a different interquartile range, so a different sized box.