Not true, each person is evacuated on severity or needs for medical care. A slightly larger person may get removed over a thinner person first, depending on the nature of injuries.

If there are two people to be got out of a dangerous situation and both have the same level of injury, they will always get out the easier one to move first. That is likely to be the slimmer, fitter person unless the fat person has to be moved so they can get at the fitter person. Once they have one out, they can give treatment to the one they have got out while working to save the other.

Yes

It comes down to triage and simple physics. Most injured/ill first, followed by the easiest to move (which enables them to treat both that person and move on to the next person).