Image Quality vs File Size: What Quality and SSIM Mean
File size is not a direct function of a quality number. At the same setting, a flat illustration may become tiny while a noisy night photo stays large; dimensions and content complexity matter too.
What a quality slider controls
A lossy encoder discards information that is expected to be less visible. A higher quality generally preserves more detail and creates a larger file, but “75” is not comparable across formats and encoders. JPEG 75, WebP 75, and AVIF 55 are not the same ruler.
Dimensions determine how many pixels must be coded. Reducing a 6000px photo to 1920px removes unnecessary pixels and gives the remaining pixels more of the byte budget. For screen-only delivery, sensible resizing can beat an extreme quality reduction.
What SSIM can and cannot do
SSIM, or Structural Similarity, compares luminance, contrast, and structure between source and compressed images. It tracks visual perception better than raw per-pixel error. tinysqueeze Smart auto performs a binary search on JPEG/WebP/AVIF to find a lower quality that still reaches a target SSIM of 0.95.
Choosing a preset
| Preset | JPEG/WebP starting point | Good for | Inspect |
|---|---|---|---|
| High quality | Quality 90 | Important photos and samples | Whether size remains excessive |
| Balanced | Quality 75 | Everyday sharing and web | Gradients and fine detail |
| Smart auto | Target SSIM 0.95 | Mixed-content batches | Local artifacts |
| Max compression | Quality 45 | Temporary delivery where size wins | Blocking and banding |
AVIF uses its own quality scale, and PNG has separate lossless and quantized paths. The JPEG/WebP numbers describe this tool’s implementation; they are not a universal cross-tool standard.
A decision order that rarely backfires
- Define the use and real display dimensions, then remove pixels that will never be shown.
- Choose the right format; do not force photos and screenshots down the same path.
- Begin with Balanced or Smart auto and inspect representative samples.
- If size still misses the target, reduce dimensions a little before crushing quality.
- Keep the important original as the master and treat the lightweight file as a delivery copy.
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