+1 on that. I have the pleasure to train fairly regularly under Murashige shihan here in San Diego and more often than not his teaching involves techniques for kuzushii from static requiring the uke to resist/grab as hard as possible. While we all may agree an attack like that is neither practical/martial, the focus is on learning proper methods of kuzushii. Occasionally a nage will resort to something else, like atemi out of frustration with a really strong uke, but that's fine with me, I stick to the program. In motion things happen too fast and other factors like uke's speed/direction come into play so it's hard to build up/understand the fundamentals.
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.