Well, the rotational moment of inertia may be smaller for a 20" wheel compared to a 27" ot 700 cm wheel.
But unless you are accelerating VERY quickly, this wouldn't be noticeable. On the same token, a larger wheel increases the "flywheel effect", and smooths out the power pulses in your pedalling.
Get closer for better drafting? Well, OK, but I don't see too many other cyclists out when I leave for work at 3:30 in the morning.
Gyroscopic effect? Not sure what this means.
Any bearing drag would have more of an effect with smaller wheels. However, it may not be measureable, but not noticeable by the rider.
Frontal area? for a 20" wheel at 1.75" wide, I have a frontal area of 35 in^2. a 27" wheel, 1" wide has a frontal area of 27 in^2. Hmmm.
With proper gearing, a 20" wheel is still very effective and comfortable. I ride my 20" wheel folder to work every day. I've taken it on 50 mile rides and it is perfectly comfortable.
I WOULD NOT ride a 20" wheel off road. I would think it takes a much smaller rut to get airborne on 20" wheels!