DEV Community

amilsam
amilsam

Posted on

Why Your WebP Is Larger Than the Original PNG

#webdev #performance #frontend

Converting an image to WebP does not guarantee a smaller file. If your exported WebP outweighs the original PNG, check the image content and encoder settings before assuming the converter is broken.

Here is a practical way to investigate.

1. Check whether the PNG uses very few colors

Logos, icons, and simple interface graphics can compress efficiently as PNG.

Converting these images to lossy WebP changes how colors are represented. That process can introduce additional color variations that are harder to compress, producing a larger file.

For graphics with sharp edges or limited colors, test lossless WebP first.

2. Separate quality from compression effort

In lossy mode, higher quality generally increases file size. However, quality 100 does not enable lossless compression.

Compression effort is another setting. Giving the encoder more processing time can improve compression without requiring you to lower the quality setting.

With Google’s cwebp installed, create two candidates:

# Lossy candidate with moderate quality and high compression effort
cwebp -q 80 -m 6 input.png -o candidate-lossy.webp

# Lossless candidate with high compression effort
cwebp -lossless -m 6 input.png -o candidate-lossless.webp
Enter fullscreen mode Exit fullscreen mode

These are starting points, not guaranteed optimal settings. The cwebp documentation explains the available controls.

3. Compare the actual outputs

Keep the dimensions identical while comparing compression. Resizing at the same time makes it harder to identify why the file size changed.

Check:

  • Downloaded file size in bytes.
  • Small text and thin lines at 100% zoom.
  • Gradients for visible banding.
  • Transparent edges against light and dark backgrounds.

For browser-based conversion, you can also use PNG to WebP Converter. Apply the same checks to its output; available controls vary between tools.

Make file size a condition, not an assumption

For a build pipeline, a useful rule is to select WebP only when it is smaller and meets your visual requirements. Retain the PNG when it produces the better result.

A successful conversion creates a valid file. Successful optimization produces a measurable improvement. Those are separate outcomes.

Top comments (0)