It is said that one of the most important principles of Ki Aikido is that "Mind Moves Body"
This is an incredibly difficult to achieve.
When you try to throw somebody with bodily force their mind is resistant to that action.
Fine if you you have the strength to overcome that resistance, but not if your partner is of equal or greater stature.
I partnered a friend of mine in his 2nd dan grading for Kung Fu.
He was 6ft plus, weighed in at 16 stone and his technique involved picking up his attacker (me!) and pile driving into the ground (using a crash mat thankfully!)
I came to the realisation at that point that hard style martial arts had it's limitation's.
Much better to guide your attacker and pursuade him to throw himself to the floor.