The 1080px ceiling nobody tells you about
Instagram resizes on upload. Anything wider than 1080 pixels is scaled down to 1080 and re-encoded as JPEG; anything narrower is generally left alone. So a 4000×3000 photo from a DSLR is stored as 1080×810, and that resized copy is the only version Instagram serves. The download you get here is that stored file, byte for byte — we don't upscale it, sharpen it or re-encode it. Any site promising to "restore HD" from an Instagram link is either interpolating pixels that were discarded years ago or lying outright. If you need the true original, the only route is asking whoever posted it.