Platforms shrink what you give them — they cannot add samples you never had
Uploaders re-encode. If you start at 640 px, the network still compresses, and the result is mush on a phone grid. If you start nearer the recommended box — 1080 × 1080 feed, 1080 × 1920 Stories, 1280 × 720 YouTube thumbs (many creators still master 1920 × 1080 stills), LinkedIn images that survive the preview crop — you have more samples for their encoder to throw away. This spoke is not a scheduler and not an API. It is a local 2× Canvas pass plus optional sharpen so you are not posting an undersized camera export. Convert HEIC first. WhatsApp will crush either way; sending as a document sometimes preserves more than a chat image. Pinterest and TikTok covers still want readable type — if type is 8 px tall on the original, scaling will not make it a headline. Cap 10,000 px, far above social needs; staying near 1080–1920 keeps files light. After scale, compress if the uploader rejects megabytes, but do not shrink below the floor you just hit. This is not 256-color GIF work.