A smaller box of lets say 2 ft by 1.5 foot means it'd be harder to move around unconsciously or be bumped around while also letting more people be on the plane. Needing to sedate yourself is an additional task on your part and it's going to be more difficult to be exact (Too much, you're still asleep by the time you're landed. Too little and you're awake before you're there. Have someone wake everyone that opted to be sedated up and you limit the majority of those issues.).