In my opinion, I can't read the developers minds unfortunately, 80 planets is the "design balance".
On a 10 planet map things go off track very fast from the developers intentions.
Yes, it is considerably harder to get knowledge and so on, but you reach the AI Homeworlds much faster so the forces the AI has built up are considerably smaller also.
It is a matter of opinion which map size is more difficult, but depending on game settings you can find a challenging game at any map size.
D.