JPG compression quality test
People open a generic “compress JPG” tool expecting a smaller file. We ran the live Compress JPG tool on eight already-encoded photographs. 8 of 8 outputs were larger than the source.
The question this page answers
If a photograph is already a web JPEG, does a quality-first compressor reliably reduce bytes while keeping the same pixels? That is a different question from “can I get under 100KB”. The 100KB question has its own experiment.
Method
Each source JPG was processed once with the public compress-jpg settings. Dimensions were left unchanged. The encoder writes a high-quality JPEG. We recorded original bytes from disk and output bytes from the generated file. No rows were discarded.
Subjects: alpine landscape, coast, forest, city, ceramic still life, books, flower, watch. Long edge about 2400 pixels. These files were already JPEG-encoded in the project library, not camera RAW.
Results
| Subject | Original | Output | Pixels in | Pixels out | Change |
|---|---|---|---|---|---|
| Alpine | 503 KB | 644 KB | 2400×1600 | 2400×1600 | +28% larger |
| Coast | 448 KB | 608 KB | 2400×1595 | 2400×1595 | +35.7% larger |
| Forest | 1.09 MB | 1.49 MB | 2400×1598 | 2400×1598 | +35.8% larger |
| City | 730 KB | 927 KB | 2400×1600 | 2400×1600 | +26.9% larger |
| Ceramic | 293 KB | 402 KB | 2400×1600 | 2400×1600 | +37.4% larger |
| Books | 406 KB | 549 KB | 2400×1551 | 2400×1551 | +35.5% larger |
| Flower | 502 KB | 743 KB | 2400×1601 | 2400×1601 | +47.9% larger |
| Watch | 106 KB | 188 KB | 2400×1744 | 2400×1744 | +78.4% larger |
What the numbers say
Every photograph in this set grew. City is the clearest example: a 730 KB source became 927 KB at the same 2400×1600 pixels. Forest went from 1.09 MB to 1.49 MB. Watch, the lightest source at 106 KB, still grew to 188 KB.
That is expected when an encoder rewrites an already-optimized JPEG at a high quality setting. The source had already spent its compression budget. The new file stores more residual detail (and JPEG artifacts of a different pass) than the compact original.
Practical conclusion
Use Compress JPG when the file is obviously heavy and you want a quality-first rewrite at full size. Use a ceiling tool such as Compress JPG to 100KB when a form names a limit. Do not treat “compress” as a promise of a smaller file.
Limits of this test
The set is eight still photographs. It does not include screenshots, documents, or illustrations. We did not sweep quality sliders. We did not score visual difference with a lab metric. A different encoder or a lower default quality would produce different bytes. The finding is about this engine, on this set, on 5 October 2026.
Technical reference
JPEG on the web is typically a JFIF file containing a lossy DCT image. MDN’s format guide is a clear overview of image/jpeg and when a lossless format is a better fit: MDN image file types.