I don't like extra room in the seat to the point that I can't keep my position secure. A lot depends on how the stirrups hangs too as to where a person's rear sits in the saddle due to where the rider's legs must be placed to reach the stirrups. It sure helps when the stirrups hang so my feet are directly below my seat. This keeps me centered and give me a more secure seat.
I prefer a high cantle myself because almost all my riding is on trails. I want a very secure seat for going up and down hills and just in case my horse decides to hop over a creek or a fallen log. A good shaped horn helps in this respect too. One that I can easily fit my hand over in case I need to grab it to stay in the saddle if my horse moves unexpectedly... and if I have enough warning to grab the horn .
"God forbid that I should go to any heaven in which there are no horses" --Robert Browning
Don't walk in front of me, I may not follow. Don't walk behind me, I may not lead. Just walk beside me and be my friend. -- Author Unknown