Expanding-Contracting Circle
The orange circles are exactly the same size. However, the one on the left appears smaller when surrounded by large circles, and larger when the surroundings shrink.
Size Constancy Failure
🤏 The Ebbinghaus Effect▼
This is a dynamic version of the classical Ebbinghaus Illusion. Your brain judges size relatively, not absolutely.
When surrounded by large objects, the center object is perceived as "smaller by comparison". When surrounding objects shrink (or move away), the center object gains "relative majesty" and appears to grow.