Originally Posted by celle

Maybe it's a question of weight. Large often equals heavy.

While never hearing the AC announcement, I have heard announcements requesting that heavy items are placed beneath the seat in front of you.

I assume this is to avoid possible injury, should a heavy item fall on a passenger. I seem to remember the case of a musician injured by some falling, heavy item from the overhead - it made the headlines at the time, but it was a while ago.