Why can two equally wide dresses with horizontal stripes look different in width when their stripe spacing differs?

Answer: The balance of dark lines and light gaps can change the width illusion.

The balance of dark lines and light gaps can change the width illusion. ✓ — The visual system judges a patterned surface, not just the dress's outer edges. Changing the spacing changes the balance of dark lines and light gaps, which can alter apparent proportions without changing the dress's actual width. Experiments with stripe patterns show that this balance affects the size illusion. Clothing design and the comparison being made still matter, so there is no universal rule that horizontal stripes make every wearer look slimmer.

The horizontal direction makes all such patterns look equally wide. — Direction alone cannot explain a difference between two patterns that are both horizontal. Their spacing and the relative widths of the dark and light bands still differ. Those features can change the visual effect, so 'horizontal' describes only part of the pattern, not everything needed to predict its apparent width.

The extra white space makes every wider-spaced pattern look slimmer. — Wider white gaps do not guarantee a slimmer appearance in every design. Changing a gap also changes the whole repeating pattern, and the effect depends on the stripe widths and the garment being viewed. A spacing that works in one arrangement cannot be turned into an always-slimming rule for all clothes.