Why A3 paper is bigger than A4?

A3 paper is larger than A4 because it is part of the ISO 216 standard for paper sizes, which defines a series of sizes based on the A series. A3 measures 297 x 420 mm, while A4 is 210 x 297 mm. The A series sizes are derived from each other by halving or doubling dimensions; A3 is double the size of A4 when folded in half along its longer side. This system allows for consistent scaling and easy resizing in various applications.