A mockup and a print file have opposite jobs. The print file has to survive a 300 DPI press on fabric; the mockup only has to look convincing on a listing page, an ad, or a marketplace thumbnail. That is why mockups can and should be compressed far harder than designs — a 4000 px mockup that a shop page displays at 800 px is carrying four times the pixels anyone sees, and it makes every product page heavier for no benefit. The failure mode to avoid is over-compressing fine detail: texture, stitching and small text turn to mush first, so compress mockups to a target you have actually looked at rather than the smallest number the tool allows. Also keep formats straight, because the two are not interchangeable here. Printful asks for PNG for transparent DTG designs and JPEG for full-coverage photographic artwork, Redbubble accepts only JPEG and PNG, Printify adds SVG and drops 300 DPI to 120-150 DPI for large-format items. A transparent PNG mockup flattened to JPEG gets a white box. Skwish keeps PNG as PNG, converts format when you ask for it, and processes a whole mockup set in the browser with a ZIP download.