That resize looks like the site’s pipeline, not your SwarmUI output. Your aspect ratio is preserved exactly: 1824 → 4096 (long edge), and 1248 → 2803 (4096 × 1248/1824), so it’s almost certainly auto-scaling everything to a 4096px max dimension and then re-encoding (often as JPG/WebP), which is where the quality loss comes from.
Quick test: download the image after upload and compare its file type/size—if it’s much smaller or JPG, it’s compression.
Workarounds: upload PNG (or max-quality JPG/WebP), or pre-upscale yourself to the site’s target size (e.g., ~2803×4096 or 2816×4096) so the site doesn’t have to resample it.
If this is Civitai, images added in some places (like descriptions) are known to get resized.
#problem with the image
astral gust
lime surge
That resize looks like the site’s pipeline, not your SwarmUI output. Your aspect...
Yes, you're right, after downloading, the image gets smaller and becomes a JPG. However, I tried generating it from JPG initially, but the site still scales it.