With little high-kv motors, the kv drops significantly under load. The 3900kv motor is bigger and lower kv to start with, so losses are very low.
Any 28mm motor will own up on all the 20mm motors.
Trust me, I thought the same thing. Check out this heavy vendetta st -
http://www.youtube.com/watch?v=eh7OJ751Vt4
Imagine that in an 18t. It makes way more power then those little 20mm motors.