Once watched many of those enormous bags get stuffed into overhead then a woman boarded with a small case and was sitting in 1st row (no under seat storage) and bins were full.
Stewardess told her she'd have to check the small bag and she didn't want to saying she had valuables inside.. Stewardess said then she'd have to take another flight if she refused to check the bag. Several people on the plane offered to have their bag checked so the lady could keep hers.
Thought that was great but the stewardess ignored the people, It almost came to a riot with people yelling that the airline should measure bags, that they saw several that are taking the space for 2 and that a solution was being offered. Of course those with (I assume) large bags yelled "check it or get off the plane so we can leave..."
Finally, the stewardess took an offered bag to check and let the lady put her small bag in the overhead. Sometimes I don't think airlines & their personel have any common sense.