File size gives it awayEvery NEF, CR3 and ARW carries a modest JPEG within it, so that the body has something to display on the back. Basic tools take that preview, log it as a recovery and go no further; a 50MB frame duly returns at 5MB.
The real difficulty is fragmentationBig files are rarely written as one continuous run. A plain carver takes the first stretch, reaches the break and gives up, which is adequate for a small JPEG and no use at all for RAW or footage. The work here follows each format’s own structure to reunite the pieces.
No two RAW formats behave alikeCR2 sits on TIFF, which makes it fairly forgiving of damage. CR3 uses a newer container that fails in different ways. Damage to a NEF shows as green and purple banding. RW2, ORF, RAF, DNG and ARW each want handling on the maker’s terms, and no one tool covers them all.
What deleting really altersOn FAT and exFAT, deleting or formatting only releases the cluster chain; nothing else on the card is altered. That is why recovery is possible at all, and it is the same reason the next exposure can land directly on the frame you want back.