Who said it had to be a laptop? It could very well be a tablet or a smartphone. I watch movies on my smartphone all the time on airplanes, for the very reason that you state: it's too cumbersome to try to use a laptop, especially during meal service. A smartphone with the right stand fits easily on or to the side of even a full meal tray.

And for those that can't imagine watching a movie on a screen as a small as a smartphone, try this at home: while seated in front of your big-screen, hold up your phone in front of your face, at about the distance it would be if it were sitting on your mail tray on an airplane. You'll probably find it's at least half as large as your big-screen TV. Try the same in a hotel room, laying on your bed - you might find that your phone is as big as if not bigger than the TV in the room. As long as the screen is high-enough resolution (such as the Retina Display on an iPhone or equivalent), your eyes won't know the difference once you start watching.