When the bike is too long, you can see that although the arms and torso are still in strong position, the hips are brought forwards on the bike - stretching the body out to a weak position where your control of the bike is reduced.
On a proportionally-sized bike, you can see that the torso and arms are in the strong, ready position and the hip has space to take a strong, ready stance position also.
On the same bike, if you come forwards to put the hip in a more natural position for aggressive riding, you can see that this then pushes the torso forwards over the stem - a very precarious position to hold.
This was the first observation. When the arm and torso are in the strong, ready position, the hip is pushed backwards, and apart from any weight distribution implications, you can see clearly that the angle is closing - this puts the hip in a weaker, less comfortable position.