Of course it is an advantage to be strong. Smaller folks with less physical power have to be better than their larger stronger counter parts do.
That said, I do think it bears some thought as to what constitutes being strong. I was training recently with Vladimir Vasiliev of the Systema. At the seminar he talked about conditioning and said that it was better to strengthen the ligaments and tendons, the connective tissue, than to strengthen the muscles. These guys are extremely relaxed but are very strong. They don't do weights but they do tons of pushups, situps, and squats, at different speeds, often slowly which is what develops the coonective tissue strength. This makes very good sense to me from an Aikido standpoint.
That said, I think it is good to remember that, according to the accounts of his students, O-Sensei was unbelievably strong in a purely physical sense. Of course there is a time when you can no longer develop that strength and have to use energy differently but conditioning is a part of that I believe. Trying to run energy through a weak structure is like trying to run 20,000 volts through a wire designed to take 12 volts.
Rather than use O-sensei's physical state at 80 yrs. as an excuse to not be as strong as possible at age 40 people should remember that O-sensei went out of his way to be as strong as he could possibly be at ecvery stage of his life.