Contrary to the popular belief that horizontal stripes make people look bigger, visual perception studies and tests of the Helmholtz illusion show that horizontal stripes actually create a slimmer and taller appearance.
The claim relies on a common cultural belief that horizontal stripes are fattening. However, empirical studies investigating visual perception and the Helmholtz illusion consistently find the exact opposite effect—that horizontal stripes can make a person or shape appear taller and thinner. Therefore, the claim is refuted by the available evidence.