Just because an airline flies big aircraft, it doesn't mean that it will have enough space for each passenger to carry a bag in the cabin. Even major names like United, JetBlue, Southwest, easyJet and Ryanair frequently ask passengers to gate check their bag if they run out of cabin space.

If an airline takes your carry-on bag at the boarding gate (a process known as "gate-checking"), it is almost always due to a lack of available overhead bin space. Most modern aircraft, particularly narrow-body jets, do not have enough storage capacity for every passenger to bring a full-sized rolling suitcase, especially when flights are at or near 100% capacity. Passengers in later boarding groups are the most likely to have their bags taken because earlier passengers have already filled the bins. Other reasons for taking a bag include the suitcase being visibly oversized according to the airline's sizer box or if the aircraft is a small "regional jet" with bins too small to fit standard carry-ons. When a bag is gate-checked, it is placed in the cargo hold for the duration of the flight. Depending on the airline and aircraft type, you will either pick it up on the jet bridge immediately after landing or retrieve it at the traditional baggage claim carousel at your final destination.