Why can continuous use of only thick lines create heaviness in a picture?

Because thick lines increase visual weight and emphasis

Line thickness is one of the properties that affects visual weight. A thick line occupies more space, produces a stronger mark, and attracts more attention than a fine line. When thick lines are used continuously around every object or in every area, the composition may lose variation and appear dense, forceful, or visually heavy. Option A is correct because it identifies the relationship between thickness, emphasis, and weight. This does not mean thick lines are always wrong; they can be useful for foreground objects, strong contours, or emphasis when balanced with thinner marks and open areas. Options B and C contradict normal visual effects, while D ignores the clear connection between line width and perceived weight.

This is a Class 9 Art Education question. Chapter: Elements of Art. Topic: Line.