The only way a passenger is going to be removed for their ACTUAL weight is if you don't meet the requirements for fitting in a single seat and haven't bought an extra seat or been upgraded. The requirements are below. 278 pounds wouldn't normally put someone in that category but, it could, depending on how the weight is carried.
https://www.united.com/ual/en/us/fly...a-seating.html
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You’ll have to buy an additional seat or upgrade if you don’t meet one of the following criteria:
- You must be able to properly attach, buckle and wear the seat belt, with one extension if necessary, whenever the seatbelt sign is illuminated or as instructed by a crew member.
- You must be able to stay seated with the seat armrest(s) down for the whole flight.
- You must not significantly encroach upon the adjacent seating space.
The 757 issue, from what I understand, has to do with the load limit of the floor in certain sections of the airplane. When the average passenger weights transition to winter weights on November 1st, you can no longer have all seats filled and still comply with that limitation. Depending on the aircraft configuration, either three or six seats, all middle seats, are blocked and unusable on the affected aircraft during the period that winter weights are used. Each blocked seat is marked as unusable and can not be occupied for any reason.
General weight restricted flights have already been discussed. In those cases, the determination is made based on average passenger weights and have nothing to do with a particularly passenger's actual weight.