Why carry-on rules vary so much between airlines

A bag that's free on Lufthansa or Singapore Airlines might trigger a £45 fee on Ryanair or a $65 fee on Spirit — the rules are set per-carrier, not by IATA or any aviation authority. Low-cost airlines have moved aggressively towards charging for anything beyond a small personal item; legacy carriers still bundle a standard carry-on into most fares. Pet policies vary even more (some cabin-allow under 8 kg, others cargo-only). Pick two or three airlines below to see their published limits side-by-side, updated from each carrier's official baggage page.