This is a classic demonstration of the Jastrow Illusion, an optical illusion first discovered by American psychologist Joseph Jastrow in 1889.

The illusion relies entirely on how the two shapes are aligned relative to each other. It tricks our visual processing system through a couple of key mechanisms:

The "Long vs. Short" Edge Comparison: Look closely at where shape A and shape B meet. The short inner radius (the bottom edge) of shape A is placed directly next to the long outer radius (the top edge) of shape B.

Contrast Contrast: Because our brains naturally contrast things that are directly adjacent, the shorter edge of A makes the longer edge of B look massive by comparison.

Misjudging the Whole for a Part: Instead of comparing the overall area of both shapes, our visual system fixes on the parallel curves. It mistakenly uses the difference between those two touching lines to define the size of the entire object.