In some of the smaller cars, that middle seat is really only big enough for a young child or a very skinny person.
We once came back from the airport 3 men and 2 ladies (me and my mother in law). One of the men drove, so where did that leave us ladies? Couldn’t let my MIL sit in the back, so I did, and spent fifteen very uncomfortable minutes trying to make sure that I was still shomer negiya. Will not do that again in a hurry.