We had a great experience with Delta! I kept my dog (14 lbs) in his carrier when we checked in and they didn't ask to see how big he was or weigh him. When I booked the trip they did ask me his weight and the weight of the carrier. The flight attendant was very kind and gave me a small milkbone treat for him when she was passing out snacks. My dog was very calm and quiet and slept most of the flight in his carrier.