Why Image Compression Results in Larger File Size: Principle of Compression-Induced Size Increase and Prevention Methods
More than 80% of users have encountered the problem of increased image file size after compression. This article analyzes the core causes and provides actionable prevention methods to help you obtain reasonable compression results.
تم التحديث 2026-08-11
Size Increase Principle of Quantization Matrix Conflict in Lossy Compression
Most web images have already undergone one round of lossy compression, and come with a visual-friendly quantization matrix. A new lossy encoder will re-encode the image with its default quantization matrix, which amplifies the high-frequency detail fluctuations of the original image.
To preserve the newly added high-frequency details, the encoder needs to allocate more coding bits to store information. This eventually results in an output file larger than the compressed source file. This phenomenon is most common in repeated JPEG encoding.
Size Increase Mechanism of High-Quality Presets for Pre-Compressed Files
Common compression tools provide different quality presets. High-quality presets correspond to smaller quantization coefficients, allowing more high-frequency details to be retained. When processing source files that are already highly compressed, high-quality presets will retain the noise-like high-frequency information generated by the original compression.
These additional high-frequency details cannot be further compressed by the encoder, and require more storage space, which eventually leads to the output file being larger than the source file. This is a normal result that conforms to compression logic.
Irreversible Size Increase Chain of PNG to JPG Conversion Back to PNG
PNG is a lossless bitmap format that stores complete pixel transparency and color information, while JPG is a lossy format that discards the alpha channel and high-frequency color information. Converting PNG to JPG and back to PNG forms an irreversible chain of information alteration.
Blocking artifacts and color shifts generated during JPEG encoding in the transcoding process are saved as new pixel information, which cannot be removed after converting back to PNG. The added noisy pixels increase the encoding size of PNG, resulting in a final output file much larger than the original PNG.
How to Identify Highly Compressed Images
The first judgment dimension is the size-to-pixel ratio. A conventional uncompressed 24-bit bitmap has a size of approximately 3MB per megapixel. If a single image has a size lower than 30KB per megapixel, it is already a highly compressed image.
The second intuitive judgment method is to check the image source. Images saved from social platforms and e-commerce platforms have all undergone secondary compression by the platform, and belong to the category of highly compressed images. It is not recommended to process them again with lossy compression tools.
| Judgment dimension | Criteria |
|---|---|
| Size per megapixel | JPG < 30KB, WebP < 20KB |
| Size ratio | Single image size is less than 10% of a RAW file with the same resolution |
| Pixel density | 300DPI resolution with size lower than the standard value for the same dimensions |
Correct Use Cases for Compression Tools
The correct objects for compression tools include RAW original images and DNG original images exported from cameras. These files retain complete original data and have not undergone lossy compression, so compression tools can achieve reasonable size reduction for them.
Another category of suitable objects is PNG and BMP original images exported from design software. These files are lossless bitmaps that have not undergone lossy compression, and can achieve stable size reduction when processed with the corresponding compression tools. Highly compressed secondary-distribution images are not suitable for repeated compression.
الأسئلة الشائعة
Can I still reduce the size of an already compressed image?
In most cases, stable size reduction cannot be achieved. If size reduction is mandatory, you can select a lower quality preset for re-encoding. This will reduce size at the cost of some image quality, and there is still a risk of resulting in a larger file.
Will processing a compressed PNG with a PNG compression tool result in a larger file?
This may occur in some cases. If the original PNG has already undergone maximum compression, the compression tool cannot further reduce redundant data, and the metadata added by the tool will cause a small increase in file size.
Will processing a compressed JPG with a JPG compression tool definitely result in a larger file?
Not necessarily. If you select a preset with lower quality than the original, you can still get a smaller file, but this will add an extra round of lossy compression and cause additional image quality loss.