Why Compressing an Image to an Exact File Size Is a Hard Engineering Problem

JPEG compression does not directly control file size — it adjusts a quantization table that discards image detail, meaning the same quality setting can produce vastly different file sizes depending on image content. A smooth sky may compress to 15KB at quality 70, while a detailed crowd photo at the same setting could exceed 180KB. The file-size-to-quality relationship is also non-linear, flattening at lower quality levels where further reductions yield diminishing returns. Binary searching the quality parameter is a common workaround but fails when images cannot reach a target size without also reducing resolution or when very low quality produces unusable artifacts. A robust solution must tune multiple parameters together — including dimensions and chroma subsampling — rather than relying on quality alone.