It may not need to be different but there are clear factors tending to make it less. As has already been pointed out, all other geometry being the same, smaller wheels will have shorter trail. And this being the folding bike forum - most 20" bikes under discussion will also be folders. Folder designers want to reduce the folded size and this will encourage steeper head angles, reducing trail and wheelbase, while increasing cockpit size. I would assume that folder designers want to increase the cockpit to wheelbase ratio for the best compromise of cockpit room and fold. There may also be the factor of the shorter head tubes in most folder designs (including BF). While I'm not sure about this (perhaps an engineer could comment), I would think a short head tube would be stressed more by a slacker head angle.