The apparent enlargement of a bright object when viewed against a dark background WordNet 3.0 & Open English WordNet

The irradiation illusion is an illusion of visual perception in which a light area of the visual field looks larger than an otherwise identical dark area. Wikipedia

Type oder illusion Wikidata

The apparent enlargement of a bright object seen upon a dark ground, due to the fact that the portions of the retina around the image are stimulated by the intense light; as when a dark spot on a white ground appears smaller, or a white spot on a dark ground larger, than it really is, especially when a little out of focus. Wiktionary