That is essentially why the Judo throw worked ...Judo forcibly takes a person's balance ...it lacks the finesse of Aikido..and Judo is largely static rondori. But I admit it showed my level of frustration that I resorted to that. Sensei told me in such a situation he would simply disengage and wait for the next attack...
I think you missed Marc sensei's point. Kuzushi is the key principle of aikido and many other MA. If you can do kuzushi from a static attack, then it will be almost effortless with a dynamic attack. Anyone (even smaller than you) can resist you with the prior knowledge of your intention; more so when you telegraph your intention by use of force. The point is about timing - he knows what you are about do but he doesn't know when you are going to do it unless you allow him to feel what you are doing. Your choice of a training partner is good and you should focus on finding the path of least resistance. Since you are a Shotokan guy like me, instead of an actual atemi as suggested by Marc sensei, try applying the principle of Hikite when doing wrist grabbing techniques.