Stronger always wins. Period.
When waza is executed by a smaller, weaker person utilizing elements that weaken the stronger person (e.g. kuzushi, atemi, timing etc.) at the point where the waza is executed the "weaker" person is in fact stronger.
Kano had a note about this somewhere online about 10 units of strength (stronger person) against 7 (weaker person) but that strength becoming 6 after kuzushi (10-4=6) making the smaller person's 7 units of power more powerful than the now off balanced, larger person's 6 units.