One photo, sent three ways.
Only one arrives untouched.
WhatsApp shrinks and re-compresses photos unless you send them as a document. Below, your photo (or a 12-megapixel test shot) goes through each route, using the sizes WhatsApp is observed to use, encoded by your browser. Compare what the other person receives.
WhatsApp's sticker spec asks for exactly 512 × 512 px, WebP with transparency, a 16 px empty margin, and at most 100 KB for a still sticker. This trims a centre square from the photo, places it inside the margin, and lowers WebP quality step by step until the file fits.
WhatsApp keeps a square profile photo and shows it as a circle. The circle hides the corners: 21.5% of the square. Here is the same crop at the sizes it actually appears.