How Image Compression Affects Quality and File Size
Learn how quality sliders, resizing, chroma detail, and output formats change the final look and weight of an image.
What compression changes
Lossy compression removes visual information that the encoder predicts people will notice less. The result can be dramatically smaller, but pushing compression too far creates blockiness, ringing around edges, banding in gradients, and muddy texture.
Quality sliders are relative
A quality value is a hint to the encoder, not a universal scale. JPG quality 75, WebP quality 75, and AVIF quality 75 can produce very different file sizes and visual results.
Resize before compressing
Dimensions often matter more than quality. Reducing a 4000 px image to 1600 px can save more bytes than lowering quality alone, and it usually looks better than keeping huge dimensions with harsh compression.
What to inspect
Look at faces, text edges, gradients, product details, and flat backgrounds. These areas reveal compression problems quickly. Preview the image at the size users will actually see.
Good defaults
For web photos, try WebP quality 80 and resize to the needed display size. For JPG, start around quality 82. For screenshots or graphics with text, compare PNG and WebP lossless before using lossy output.